Long boards have a pretty good tendency to warp if not supported correctly.

My bought new from the store Voodoo 5 5500 is pretty warped and it still works fine. Part of the warping is due to me installing large socket 7 heatsinks on it shortly after I got it.

Traces expand with heat and shrink with cold, so they won't care much about some warping of the pcb. I have some Mac PPC CPU cards which are heavily bent due to their heatsink, but they still work fine.

I've an old dual socket 771 board that, despite the bracing support of the CEK spring and backplate, has warped around the CPU1 socket to such an extent that the board only recognises a processor in this socket when a heatsink is installed and fully loaded (probably overloaded TBH). Could've caused a slight pin mis-alignment or solder joint / trace problem, but strangely no stability problems.