...For all intensive purposes". I don't know why the fact that it makes no sense doesn't tip people off that they're using this incorrectly. The correct phrase is "for all intents and purposes".
...
There's probably a lot more, but I can't think of them off the top of my head.
How about "commonal garden" for "common or garden" or "common, garden-variety..."? Again, the fact that "commonal" is not even a word doesn't seem to figure in people's use of the phrase.
Cheers, Al'Kelhar