Please state in which country your phrase tends to be used, what the phrase is, and what it should be.
Example:
In America, recently came across “back-petal”, instead of back-pedal. Also, still hearing “for all intensive purposes” instead of “for all intents and purposes”.
I feel that this one is slightly pedantic because, strictly speaking, “the proof is in the pudding” is also technically correct. After all, the proof of the pudding is in the pudding. Yes, the more correct form is much more clear as to what it means, but that doesn’t invalidate the mis-phrasing.