On another thread a poster has used the term 'weigh station' instead of 'way-station'. The poster is obviously intelligent and can write, this has nothing to do with that. But it's curious how few people think about the meaning of the idioms we use every day. Clearly a 'way station' is a station along the way; if a 'weigh station' existed, it would presumably be where things are weighed, which makes no sense in context.
Another common one is 'give free reign' (some weird royalist/anarchist fusion?) in place of the correct 'give free rein' (as in, give a horse freedom ot run where it likes instead of reining it in).
"Plumb crazy", I'm guessing, means 'crazy to the very depths of one's being" (the reference being to the dropping of a plumb-line in surveying); the incorrect 'plum crazy' would seem to refer to a passion for purple fruits.
Unfortunately, once you direct your attention to these kinds of mistakes you see them popping out everywhere. It's enough to drive one plumb crazy.