Riding into work today, I got about half way in before noticing that my rear shifting wasn't working very well. Looking down, it looked like a rag had got caught in the jockey wheels/cassette.
So I stopped on the Moonee Ponds Creek bridge and had a look. It turned out to be a single sheet of damp newspaper, all caught up and mashed around the jockey wheels & cassette & under the chain.
Cleaning it out was difficult because it was caught up under the chain, and that it disintegrated when picking it out. Took about 5 minutes to get it out but then I noticed that the cassette, the paper had somehow loosened the locknut on the cassette and the cassette & the separate sprockets were very loose. Tried finger tightening the locknut, but that only worked a little bit, it was still loose.
Seems that the paper must of compressed on each rotation, in the gap between the locknut & chainstays. And since the chainstays are "going backwards" in relation to the clockwise drive of the cassette when pedalling, it unscrewed the locknut!! Who would of guessed, I'm going to make it nice & tight when I get back home to my cassette locknut spanner.
PS. I've heard some horror stories of plastic shopping bags wrecking the rear derailleur, so I got away pretty cheaply, especially since I just replaced my Shimano Deore 2001 rear derailleur with a SRAM X.9 2007 one.