Users can only upvote five questions per proposal on Area 51, as is well known. It is probably less well known that you don't have to have followed a proposal to upvote or even propose questions.

What I didn't know until I tested it today is that there does not seem to be any limit in the number of downvotes a user may inflict on a proposal, even one they never committed to. I just successfully made (and then reversed) seven downvotes on questions on a proposal I am not following.

Bulk downvoting would probably be picked up by the abuse detectors but it does seem like a loophole that would allow users to throw sand in the wheels of a rival proposal to the one they favor, if that hypothetical situation were ever to arise