Today I wrote a random comment on somebody's LJ, and five minutes after I posted it I suddenly realised I'd inadvertently used the phrase ‘sounds good on paper’. Not sure how something does that. Does it rustle pleasantly, perhaps?
I suspect that mental crossover was simply due to my brain being momentarily indecisive between ‘sounds good’ and ‘looks good on paper’, and the fact that what I was thinking was an entirely abstract thought about the superficial plausibility of the comment I was responding to, to which either of the phrases I was considering would have been at best an approximation.
A more interesting case of linguistic synaesthesia showed up in a mathematical proof I jotted down in 2001 and recently found lying around on my computer, which described a nasty algebraic mess as ‘the following smelly-