I've noticed that if you use a the flying saucer and you get a crate while flying it that fits the F4 slots, that weapon will be selected automatically instead of having the saucer selected.

When playing shoppa mode with flying saucer instead of ropes (which I played yesterday with nemo) and collecting a Whip/Fire Punch/Kamikaze (worst) crate will be selected immediately after saucer fuel is consumed leaving you executing one of those weapons instead of flying a new saucer if you aren't lucky enough to hit F4 until the saucer is reselected again.