When your troubleshooting procedures remind your customers of their (erroneous but widely practiced) methods of making NES cartridges work, you might have a problem.
I don't know about NES, but at least with the N64, I swear by blowing into the cartridge. Works at least 75% of the time, and the other 25% by wiggling the cartridge a bit in the hopes of getting it to reseat better.
It works -- but what you're really doing is fixing an intermittent electrical connection by introducing moisture. Over the long term that's a recipe for corrosion.