My son has a "Eurobike" (hybrid/"comfort" bike with Nuvinci variable speed rear hub and fully-enclosed chainline). He broke a spoke (which is another discussion) and since the spoke broke at the nipple we had to dismount the rear wheel to replace the nipple.
Anyway, this is the first time any of us (two grown sons and myself) had dealt with such an arrangement, so a little fumbling was exspected. We removed the two screws securing the cover over the hub area and, after some fiddling, pulled out the wheel. Replaced nipple and spoke, did some "by guess and by gosh" wheel truing, and went to reinstall the wheel.
Well, long story short, somewhere in the process of getting the wheel back in the chain jumped off the front sprocket. This created a royal PITA situation in the relatively controlled circumstances where we were (concrete surface in the shade at a camp site) but would have been a total disaster if repairing a puncture on the road.
The remaining 5-6 (tiny, easily lost) screws securing the chain cover were removed, but it would not come off due to the crank arm blocking it. So it was a matter of the three of us fishing for the chain with the broken spoke (while prying the halves of the cover apart) to get the chain back on the sprocket, a process that took an extra 10-15 minutes.
Granted, inexperience was a factor, but it would seem that there's got to be some "trick" to keep the chain in place, other than to just hope to have better luck next time.
Does anyone have experience with this and know what could be done differently?