I had the same exact problem with my 01 C5. It would go past 5th into the reverse slot, and it would only do it when I was driving it hard, and my dumb a$$ kept pushing it up. I told my service advisor about it, but he wouldn't do anything about it. There were no codes, and they weren't able to reproduce it.
It happened to me 4 or 5 times over a 3 year period, and once the warranty ran out, the trany started acting up on me. It first started to grind going into reverse, and then the same thing with first.
Anyway ended up costing me $5K to replace the transmission.
I was hoping that they had found and fixed this problem since then. I guess I am glad I got the 100K mile warranty on it.