What is sandbagging in costuming?
It is when someone competes a costume at a larger convention, wins an award, and then takes it to a smaller convention and competes it there. This is a practice highly frowned upon by costumers.
In fact, when some masquerade directors find out this is the case, you are not allowed to win any awards at that masquerade, and you get a bad reputation. People are encouraged to exhibit their award-winning costumes at the smaller convention. Exhibiting a costume means presenting it without competing against any of the other entries.
Note
If you have a costume that you compete at a smaller convention, and you want to compete it at a larger convention, please do so. This is considered a way of doing a test run for the larger convention, and is an accepted practice.