The learning curve! After spending three hours looking for a 11009-1377 spark plug well seal I lost while the valve cover was off for a valve adjustment, I decided I would never let that happen again!
I found an OEM source and now carry them!
And another thing, if you find oil showing up in your spark plug wells while just doing a routine check or change of the spark plugs, this is where it's coming from. These things can and do lose flexibility over time, and particularly if your machine is one with some years on it. It also doesn't hurt to be prepared ahead of time when planning a valve adjust!
Believe me, these are very much a "it's better to have it and not need it, than to need it and not have it" part!