Good point Eric, I always forget that one. I cured my intake manifold problem permanently with thin, non-compressible gaskets, heavy steel backing washers, and those special long steel 11mm nuts on the studs. I also use Loctite 517 anaerobic sealant and primer on both sides of the gasket. Never a problem for me. They came right off too, but were still sealed after over 30,000 miles.
The thick gaskets some vendors sell will squish out eventually, causing a vacuum leak.
A vacuum or exhaust leak can cause the backfiring you're experiencing.