diff --git a/doc/release-notes/rl-2511.section.md b/doc/release-notes/rl-2511.section.md
index a62e6bc0c001d..e538373e8e2c6 100644
--- a/doc/release-notes/rl-2511.section.md
+++ b/doc/release-notes/rl-2511.section.md
@@ -176,8 +176,13 @@
 
 ### Deprecations {#sec-nixpkgs-release-25.11-lib-deprecations}
 
-- Create the first release note entry in this section!
+- `types.either` silently accepted mismatching types when used in `freeformType`. Module maintainers should fix the used type
+  In most cases wrapping `either` with `attrsOf` should be sufficient.
 
+  Since types.either was used to bootstrap other types. This also affects the following types:
+  - `oneOf`
+  - `number`
+  - `numbers.*`
 
 ### Additions and Improvements {#sec-nixpkgs-release-25.11-lib-additions-improvements}
 
diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index fbbccd8172f67..3b402b7d856cd 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -550,6 +550,28 @@ checkConfigOutput '/freeform-submodules.nix"$' config.fooDeclarations.0 ./freefo
 checkConfigOutput '^10$' config.free.xxx.foo ./freeform-submodules.nix
 checkConfigOutput '^10$' config.free.yyy.bar ./freeform-submodules.nix
 
+# Regression of either, due to freeform not beeing checked previously
+checkConfigOutput '^"foo"$' config.either.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong.nix
+NIX_ABORT_ON_WARN=1 checkConfigError "One or more definitions did not pass the type-check of the \'either\' type" config.either.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong.nix
+checkConfigOutput '^"foo"$' config.eitherBehindNullor.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong.nix
+NIX_ABORT_ON_WARN=1 checkConfigError "One or more definitions did not pass the type-check of the \'either\' type" config.eitherBehindNullor.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong.nix
+checkConfigOutput '^"foo"$' config.oneOf.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong.nix
+NIX_ABORT_ON_WARN=1 checkConfigError "One or more definitions did not pass the type-check of the \'either\' type" config.oneOf.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong.nix
+checkConfigOutput '^"foo"$' config.number.str ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong.nix
+NIX_ABORT_ON_WARN=1 checkConfigError "One or more definitions did not pass the type-check of the \'either\' type" config.number.str ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong.nix
+
+checkConfigOutput '^42$' config.either.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong2.nix
+NIX_ABORT_ON_WARN=1 checkConfigError "One or more definitions did not pass the type-check of the \'either\' type" config.either.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong2.nix
+checkConfigOutput '^42$' config.eitherBehindNullor.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong2.nix
+NIX_ABORT_ON_WARN=1 checkConfigError "One or more definitions did not pass the type-check of the \'either\' type" config.eitherBehindNullor.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong2.nix
+checkConfigOutput '^42$' config.oneOf.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong2.nix
+NIX_ABORT_ON_WARN=1 checkConfigError "One or more definitions did not pass the type-check of the \'either\' type" config.oneOf.int ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong2.nix
+checkConfigOutput '^42$' config.number.str ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong2.nix
+NIX_ABORT_ON_WARN=1 checkConfigError "One or more definitions did not pass the type-check of the \'either\' type" config.number.str ./freeform-deprecated-malicous.nix ./freeform-deprecated-malicous-wrong2.nix
+# Value OK: Fail if a warning is emitted
+NIX_ABORT_ON_WARN=1 checkConfigOutput "^42$" config.number.int ./freeform-attrsof-either.nix
+
+
 ## types.anything
 # Check that attribute sets are merged recursively
 checkConfigOutput '^null$' config.value.foo ./types-anything/nested-attrs.nix
diff --git a/lib/tests/modules/freeform-attrsof-either.nix b/lib/tests/modules/freeform-attrsof-either.nix
new file mode 100644
index 0000000000000..3d6ea05a88efc
--- /dev/null
+++ b/lib/tests/modules/freeform-attrsof-either.nix
@@ -0,0 +1,14 @@
+{ lib, ... }:
+let
+  inherit (lib) types mkOption;
+in
+{
+  options.number = mkOption {
+    type = types.submodule ({
+      freeformType = types.attrsOf (types.either types.int types.int);
+    });
+    default = {
+      int = 42;
+    }; # should not emit a warning
+  };
+}
diff --git a/lib/tests/modules/freeform-deprecated-malicous-wrong.nix b/lib/tests/modules/freeform-deprecated-malicous-wrong.nix
new file mode 100644
index 0000000000000..4d1e86cf5ae1e
--- /dev/null
+++ b/lib/tests/modules/freeform-deprecated-malicous-wrong.nix
@@ -0,0 +1,18 @@
+# Obviously wrong typed
+{
+  config.either = {
+    int = "foo";
+  };
+
+  config.eitherBehindNullor = {
+    int = "foo";
+  };
+
+  config.oneOf = {
+    int = "foo";
+  };
+
+  config.number = {
+    str = "foo";
+  };
+}
diff --git a/lib/tests/modules/freeform-deprecated-malicous-wrong2.nix b/lib/tests/modules/freeform-deprecated-malicous-wrong2.nix
new file mode 100644
index 0000000000000..62c2a7befeb5c
--- /dev/null
+++ b/lib/tests/modules/freeform-deprecated-malicous-wrong2.nix
@@ -0,0 +1,19 @@
+# freeeformType should have been (attrsOf either)
+# This should also print the warning
+{
+  config.either = {
+    int = 42;
+  };
+
+  config.eitherBehindNullor = {
+    int = 42;
+  };
+
+  config.oneOf = {
+    int = 42;
+  };
+
+  config.number = {
+    str = 42;
+  };
+}
diff --git a/lib/tests/modules/freeform-deprecated-malicous.nix b/lib/tests/modules/freeform-deprecated-malicous.nix
new file mode 100644
index 0000000000000..47d5bd2f69dd7
--- /dev/null
+++ b/lib/tests/modules/freeform-deprecated-malicous.nix
@@ -0,0 +1,34 @@
+{ lib, ... }:
+let
+  inherit (lib) types mkOption;
+in
+{
+  options.either = mkOption {
+    type = types.submodule ({
+      freeformType = (types.either types.int types.int);
+    });
+  };
+
+  options.eitherBehindNullor = mkOption {
+    type = types.submodule ({
+      freeformType = types.nullOr (types.either types.int types.int);
+    });
+  };
+
+  options.oneOf = mkOption {
+    type = types.submodule ({
+      freeformType = (
+        types.oneOf [
+          types.int
+          types.int
+        ]
+      );
+    });
+  };
+
+  options.number = mkOption {
+    type = types.submodule ({
+      freeformType = (types.number); # either int float
+    });
+  };
+}
diff --git a/lib/types.nix b/lib/types.nix
index 6b51f9254a00c..c7136de7f2a63 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -1453,7 +1453,12 @@ let
                       headError = {
                         message = "The option `${showOption loc}` is neither a value of type `${t1.description}` nor `${t2.description}`, Definition values: ${showDefs defs}";
                       };
-                      value = abort "(t.merge.v2 defs).value must only be accessed when `.headError == null`. This is a bug in code that consumes a module system type.";
+                      value = lib.warn ''
+                        One or more definitions did not pass the type-check of the 'either' type.
+                        ${headError.message}
+                        If `either`, `oneOf` or similar is used in freeformType, ensure that it is preceded by an 'attrsOf' such as: `freeformType = types.attrsOf (types.either t1 t2)`.
+                        Otherwise consider using the correct type for the option `${showOption loc}`.  This will be an error in Nixpkgs 26.06.
+                      '' (mergeOneOption loc defs);
                     };
               in
               checkedAndMerged;