Mezzmo 3.4.4.0 on Windows XP:

Smart playlist with:

Match all of the following rules:
Genre contains Comedy
Genre contains Romance
Album/Series is (blank)

shows nothing (expected to show romantic comedies that are not series).

Smart playlist with:

Match all of the following rules:
Genre contains Comedy
Genre does not contain Drama
Genre does not contain Romance
Album/Series is (blank)

shows an apparently random subset of comedies that are not series (expected to show comedies that are not also dramas, romances or series). Everything in the list contains Comedy in the Genre; some contain Drama, some contain Romance, some contain both, some contain neither, and some movies that contain only Comedy in the Genre are not on the list.

What am I missing?