I've been keeping track of nouns I've come across which are semantically singular but syntactically plural, just because I think it's a cool quirk of the language. These are the ones on my list. Do you know of any others?
castra -orum
cunabula -orum
epulae -arum
fauces -ium
idus -uum
indutiae -arum
insidiae -arum
kalendae -arum
nonae -arum
castra -orum
cunabula -orum
epulae -arum
fauces -ium
idus -uum
indutiae -arum
insidiae -arum
kalendae -arum
nonae -arum