Rule: Your team must not use the same weapon twice in a row (e.g. if you used a bazooka during the previous turn, use something else).
What happens if I decide to annoy my opponent and cow?: If you break this rule and kill any enemy worms, kill the same number of yours; if you didn't kill any worms, just skip. Not doing the penalty will cause your opponent to win.
Skipping penalty does not count as a weapon: if you had to skip e.g. because you used a banana bomb twice, you still have to use something else after skipping.
You can get some maps from here: http://wmdb.org/maps/?map[type]=plopwar