I have a 13-26 9 speed Campy cassette and Campy 9 speed rear derailleur. When I'm in the 23 cog (the second lowest gear), the teeth of the 26 cog hit the top of the inner cage. It's a short cage rear derailleur which officially should be able to handle a 13-26. I've already got the body tension screw all the way backed out. This is a later generation Campy RD whose "b-tension" screw for moving the top pulley away from the cogs is to turn the screw counter-clockwise.
I posted this in another forum and the best response I got was to shorten the chain. This suggestion worked, but I had to shorten the chain by two links. The result is that I now need to be very careful about accidentally shifting into the big-big combination.
Any other thoughts?