I think redefining volatile wouldn't be harmful enough because so few people use volatile correctly anyway. By contrast, the break/continue or double-free suggestions would show up too quickly. I don't think I can come up with anything as insidious as some of the "one in a million" suggestions, but I like this one for its brevity.

#define do

You could even slip that into a makefile (-Ddo=) and it might not be noticed.