It goes without saying that one should always be on time for concerts if one is performing.
I usually aim for arriving at the concert venue and being ready to play 30 to 60 minutes early, and also add 50% onto my expected journey time. I've not yet been late for a performance.
At Sinfonia on Thursday night we had a problem. Our first horn player was stuck in traffic. This wasn't a huge problem for the Biber, because it has no horns in it.
The Vaughan Williams Folksong Suite does have four horn parts, but they are printed with 1st & 2nd on one set and 3rd & 4th on the other. Since we each had a copy of a part, all the parts were there. It is possible to cover most of the harmonies with three horns, because the work is quite heavily scored with lots of doubling. The third player jumped up to first; I played the third and fourth parts, alternating based on my own knowledge of the piece and the other parts. This is really not ideal but I doubt anyone in the audience noticed, except for the empty seat beside me.
The Mozart was going to be problematic, even though it has fewer horn parts. There was only one copy of the first horn part, and it was with our delayed first horn player. I hadn't been scheduled to play in that piece at all, though I believe I have performed it before. The first part and the second part are fairly similar, it's basically all octaves all the time except for a few bars in the last movement, but those few bars are fairly crucial. We had a very quick look at the score and decided that the thing to do would be to have the second player jump up to first (since he'd been rehearsing and knew what to expect), and let me sightread the second part.
Thankfully it didn't happen that way. We were in our seats and about to tune when our first horn player arrived, flustered and rushed but with her horn and her part. I must admit I was almost disappointed as it would have been fun flying by the seat of our pants like that, but it's probably for the best.
This is what happens when you are late for a performance: other people have to take big risks. That is why it is considered so awful. Every part really is crucial. And what would have happened if I did not know the Vaughan Williams so well, or if our first player had not made it in time for the Mozart and there were no other horn players available to try to cover the part?