An eggcorn is a special kind of malapropism that makes a certain amount of sense. The word "eggcorn" is itself an eggcorn of "acorn"—because an acorn is shaped something like an egg. Other examples include:
old-timers' disease instead of Alzheimer's disease
feeble position instead of fetal position
self phone instead of cell phone
internally grateful instead of eternally grateful
rebel rouser instead of rabble rouser
cold slaw instead of cole slaw
To see (and contribute to) a long list of eggcorns, visit the Eggcorn Database.
Are there any eggcorns that you use or notice? Please share!