From Frank Mayer:
[referring to playing a piece] If a key part (e.g., a solo) is missing, it might be covered by another (e.g., a drum covering for a missing cymbal). However, by the time the blank is detected, any "cover" would be late--Better late, or never?
In general, the section tries to cover all of the parts. But, as Frank says, there are times "on the go" when a section member might realize that there is a part coming up that isn't going to be covered. Ideally, the player will try to play something at the correct moment: The correct instrument would be best, a similar instrument would be the next best, a totally different instrument would be okay if it helped the ensemble realize that there is some sort of sound that fills that potential empty space--especially during sight reading when everyone is trying to hear how a piece goes. BUT--to answer the question, I would pick never instead of late if you realized the "gap" too late to play it at the correct time. To play something out of time disrupts the flow of the piece and also gives a distorted view of how the piece goes. It is better to let it go and remember the spot for the next go around instead of disrupting that one! Also remember, if you are in a shorthanded section, that you can pick out the most important parts and let some parts go--especially if they are covered in another part (e.g., a woodwind part is already playing a mallet part). It isn't ideal, but sometimes there are only so many people and too many parts.