You can presently edit the Samples on a read-only clip in the Animation Window. This should not be possible, since the clip asset is not editable and the edit will be lost.

repro:
-Add attached BallAnim.fbx or any fbx asset with animation
-Drag Ball asset into scene
-Drag animation clip from within asset onto Ball GO
-Open Animation Window
-Scrub timeline to activate clip
-Change the Samples to 12 (or any other sample rate)...
Bug: Clip is resampled & error is thrown...

Curve is not editable and shouldn't be saved.
UnityEngine.Debug:LogError(Object)
UnityEditorInternal.AnimationWindowState:SaveCurve(AnimationWindowCurve) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimationWindowState.cs:375)
UnityEditorInternal.AnimationWindowState:set_clipFrameRate(Single) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimationWindowState.cs:1129)
UnityEditor.DockArea:OnGUI()

Expected result: Clip should not be resampled since it is a read-only clip.

This is not a regression, but error is only present since 5.4
Before 5.4, the same problem occurs, but there is no error message.

This is repro in all versions including 5.6.0a3, 5.4.3f1, 5.3.7f1