Oh, this brings back a bad memory..... (though I was only performing with 1 violinist, which isn't quite the same as an entire orchestra)
This year I was asked to perform a violin/piano duet for Mozart's 250th birthday. It was pretty easy, only about gr. 5 or 6 (RCM). I hadn't really practiced the day of the concert, and 3 parts I totally screwed up, and t was only a 2
1/2 page song!
The first time I messed up I had forgotten the repeats, but the violinist kept going, so I just faked a bit and I got back on track. The 2nd time I wasn't so lucky though. After we had repeated the part I messed up, it went into the alternativo part. But no, I had to get nervous, and I repeated the section for a second time! woops! I had even forgotten what my part sounded like. (I sometimes got confused because it almost looked and sounded like I should be playing triplets, but it is just regular 8th notes) I just stopped playing, whispered quickly to the violinist to start from te pick up (which I had forgotten I had too). Nevertheless, we continued playing. Then, less than half a page later, I had a 2
1/2 octave jump (though it was slower than the rest of the song), and I totally missed the notes, got flustered (for the umpteenth time), forgot where my next few bars lined up with her violin part. I stopped again, pretending she had a short 2 bar solo, and then I picked up!
So you know what, if you make a mistake, just pretend that it's supposed to be that way!

Good luck, I hope you make (made?) the audition and play well