This is mostly a curiosity question as replacing the spark plugs solved the problem.
I have a mercury 40HP outboard two stroke motor. It is a twin cylinder and after running it out of gas it began to have a hard miss on one cylinder. I diagnosed it to have fuel (plug was wet), spark (very strong, blue spark, jumping significant distance), and air/compression. However, the one cylinder continued to miss until I swapped plugs - the misfire followed the bad plug. A new set of plugs solved the issue
I'm curious as to what would cause the plug to spark very reliably in free air, but as soon as I install it in the cylinder it begins to misfire? The plug was clean (cleaned from all the extra gasoline on it) and working fine. Ohm checks showed the coil is undamaged.
If it's relevant - these are surface discharge plugs: NGK BUHW