Nes4life said:There's a very quick vertical line on the odd explosion but everything else is perfect.
That's pretty much normal. It's not correct, obviously but it's so common as to not be worth worrying about. It usually happens when one or more of the 191 counters in the X/Y stage just before it goes to the DACs (sorry, I forget the name) have been replaced and so you have a mix of different manufacturers. Replacing all 6 of the chips for new ones of the same make usually fixes it.
That's in my experience anyway. It can be other things too I'm sure. Or could be one or more of them going bad.
But bottom line, ignore it unless it gets worse.