(and singulars) These tend not to be used or to be misused.
fora (not forums!!!) *also, not to be confused with flora.
fishes (fish as a plural doesn't work when there are multiple species of fish)
corpora (like if there was another city somewhere named Corpus Cristi, that and the TX one could be corpora Cristi??)
genera (plural of genus)
media (lots of people mean medium)
and i just like nuclei
datum (lotsa people use data like it's singular)
elf (which is plural in German...)