The craziest costume that I ever saw, was when a group of six us dressed up as a caterpillar. We had a sheet of plastic that was about 12' x 12'. We had spray painted it with yellow, green and red circles and cut out holes for our heads and our hands.
We then painted our faces green, and cut off the bottom of egg cartons, where the eggs sit, painted them green, and cut out holes for our eyes. We attached fake eyelashes to that, and then, affixed them to our faces with false eyelash adhesive.
We took snowmobile helmets and wrapped them in Green Foil wrap, and glued on black pipe cleaners with white ping pong balls attached. We attached one piece of paper to the head-head, and had listed a list of drinks for us - none of them our regular drink so, that we wouldn't be recognized.
Then, using two different cars, we headed out pub crawling. Outside of each pub we would put on our "plastic" and head in. We couldn't sit together, but, we just walked around. Finally, one of the inn keepers put a board over the pool table and set up some chairs, where we could all sit.
Worst part - when, we had to use the ladies room - we all had to go to the ladies room! We all had to get in the stall either all the way or part way, and one would use the facilities and then, we would try to rotate around to get out of the stall.
Best part - we won 2 first prizes at 2 different pubs. It was a great time!