Loose cassette on road bike I recently bought a new wheel for my racing bike. The first few rides were in rainy weather so I took the cassette off to clean it. When I put it back on I noticed that there was a gap of several millimetres between the outer sprocket and the lock ring. As a result the two outer sprockets (the ones separate from the rest of the cassette) wobble quite noticeably when I rotate the wheel. The cassette was fitted by the LBS where I bought the wheel. It is 8 speed (Shimano) and has 12 teeth on the smallest sprocket and 25/26 on the largest.

I have another 8 speed cassette on my bybrid bike. That had 32 teeth on the large sprocket and fitted just fine when I put it onto the racing bike. Likewise both cassettes fit ok on the wheel of the hybrid bike.

Having looked at other answers on this site, it seems like the most likely explanation is that I mislaid a spacer when I removed the cassette. Does that seem right, or is there another possible explanation which I have missed.
Seems like I'm a Necromancer again...

Most likely a spacer. No, I take that back; Almost certainly a spacer. With the Hybrid cassette fitting correctly, it's quite often that they are slightly thicker, even by a couple of millimeters. An easy way to check is to check the limiter screws on the deraileur if you haven't adjusted them. If there's too far away from the wheel for where the jockey wheels line up, it's probably missing a spacer. If not, it needs a thicker spacer or more, assuming the gears were properly set up before you took it to pieces.

Also, good call, taking the cassette off to clean it. Too many times, people leave it on their wheel using a degreaser to clean it, and it breaks down the grease inside the hub and freehub body! Annoying because I usually get asked to replace them when it happens, then I lecture them about all the problems from doing so..... Boring!