We could probably get the answer by hanging out on an ESL forum for a bit, that's assuming it's non-native English speakers who are making this mistake (it might not be!).
I suspect it's almost entirely native English speakers making this mistake. Those who learn their English as an acquired language are taught about this sort of thing, and it's known intellectually.
Native English speakers just rattle stuff out, and don't know any explicit rules (and their exceptions) to go back, re-read, and check.
Native English speakers just rattle stuff out, and don't know any explicit rules (and their exceptions) to go back, re-read, and check.