Quoted from Pin_Guy:

This is what I find on the vast majority of sticking flippers on stern machines...I rarely ever use a rebuild kit as these fail LONG before the plunger ever shows any sign of wear; although I do replace the coil sleeve every time because its a cheap trow away part and there is no extra work involved with replacing it.
The dead giveaway here is is you see a little collection of brass dust at the bottom of the cabinet the coil stop is going to be bad.

No bet ... but not from mushrooming, the brass retaining rivet will never last long enough for that to happen.