[dafucwuzat]

Pulled drive and splines are great on both ends. Early last season, small debris in water, boat settled into water but rpm went up. Thought wood jammed in the props. (Bravo3) Nothing found. Took off and did fine rest of season, but our season was limited this year. Anyway, I can't find anything wrong. With that said, I found motor mounts backed off when winterizing. So, alignment off. Now to the question, can the coupler slip internally rather than splines. I thought it was solid metal connection but not sure.

[Motoxxx43]

A bad alignment will wear it out fast but I don't see it slipping only a little or once in a while. Coupler bolts to flywheel. Coupler splines meet drive shaft splines. No other splines to question. Its possible a random object caused the prop slip. I've seen that happen on other boats an had it happen to me.
Prob not a bad idea to grease that shaft well.

[ShabahZ280]

I'm with motoxxx43 here. It's a solid connection with just the splines of the drive and the coupler. It wouldn't slip once in a while, it goes once and that's it. A bad alignment will tear that coupler up quickly, so I'd get that alignment fixed, and maybe consider replacing the coupler if the engine's out!
Had two couplers go, one in the Z250, and one in the Z280 two years ago. Not something that's fun to experience on the water. I check and grease mine regularly now.