If you /hmake a hash table with a name of more than 256 (2^8) chars, the tablename is truncated silently. I think it should throw an $error.
This limit is not imposed by mIRC itself I suppose (?), but 256 is far from the limit I expected (viz. the "usual", internal ~4100) and other length limitations do throw an error if they're hit.

(This may as well be a feature suggestion)