Anybody ever cop BNB on an exchange and only a portion of the order fill?
sure that it possible.
there is a matching engine which matches and runs outstanding bids together against outstanding asks.
it's possible (especially in illiquid markets) that there is no match.
also it is also possible that you cannot represent one or the other exactly in the opposing currency (token) and so you can end up with small part values known as "crypto dust".
nice exchanges like binance will hold unfulfilled part orders open or will allow you to convert your dust (into BNB).
that's generally why it is better to trade in liquid markets and on nice exchanges.
or yeah, your bid/ask might be too far away from the current price