Yeah, I think the sidestand thing is pretty common. Usually it's located where it'll get sprayed with chain lube. Between heat cool cycles, vibration, and constant oiling, it usually finds a way to come loose.
The front sprocket: probably better explained with pictures. Apologies if this is a bit kid's primer, a few people have struggled with what's going on here so I'll break it down as far as possible. Let's go...
1) nice new sprocket and locking tab washer. The washer is set up so it'll slide onto the gearbox output shaft, notice how the spline teeth are lined up and the screw holes are covered.
2) locking tab washer is now set up as it would be once everything was on the output shaft. The spline teeth are now one step out of phase.
3) munted locking tab washer (the problem) now in place, notice how the locking teeth that should be there just aren't any more. They've been completely worn away.
4) the gearbox output shaft, where the front sprocket attaches. Note the groove near the shaft end. I've slid the locking tab washer on past the point it'd normally lock to.
5) Locking tab washer in its normal position, teeth in the groove, teeth rotated so that it won't slide on the spline any more. This isn't normal, usually the front sprocket would be underneath this.
6) Front sprocket in place, locking tab washer in place, screws not yet properly tightened up. When they are, the sprocket will be pulled into its proper position on the output shaft. The sprocket can't slide further down, or off, the output shaft. The locking washer is doing its job and locking the sprocket to the shaft groove.
And I've run out of pictures... The problem on my bike was that the teeth were completely gone off the locking washer. The whole assembly was free to slide on the output shaft's spline. It wasn't bad assembly. It's pure neglect and wear.
The cause: the front sprocket and the output shaft splines aren't an exact fit. They're just a little bit loose. They have to be, otherwise the slightest bit of dirt between them and you'll need a puller to yank the sprocket off (maybe that happens sometimes, I don't know). Anyway, slightly loose meant a slight motion per rotation was possible. Movement plus grit means abrasive wear. After a while, the locking teeth are simply gone. Ducati specify a check / replace interval of every 10,000 kms... I think there's a pretty good chance that this is the original washer and it's never been checked properly.
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