High speed miss or worse. Can cause overheating, since you aren't getting all the power out of the fuel charge. Weak spark causes poor flame front propagation.

Saw this on my son's old Volvo. Gaps were .010 wide, and the car ran dirty for the smog check, started hard, lower fuel mileage, etc.

I screwed up and put a set of plugs out of the box into my 428CJ (thought I'd gapped them - gapped the other set on the shelf instead), and got missing and so on at about 3500. Fixed the gap, pulls to 6200 easy.