Not possible to get an 18 stat or +3 modifier?

When leveling up, the rules read that at 3rd, 6th and 9th level, you get +2 to a single stat, but a stat can't go above 18.

Given the starting spread (17, 15, 13, 11, 9, 8), seems like an 18 won't ever happen?

Or, if my 2nd level (near 3rd) fighter wants to add 2 to his 17 STR, does it go to 18 and he loses one point of that stat raise?

When one of my players reached 9th, they took this advance and it took his thief's Int to 18 with one point left ofter. We wondered what to do and decided it was an untapped authoring resource we named moment of clarity. He hasn't used it yet, but he has the one off custom move; that when all the pieces fall into place he may reveal his moment of clarity. He may introduce
*a major situation or stake to be resolved
*an Major NPC or Danger and their instinct
*a Steading of his choosing and location with a tie to his character

That's a great idea, giving a one time in-fiction benefit like that. Thanks for the idea.

Cool custom move!

Also, the numbers maybe need to be tweaked in the actual game.