I used to have the same guy come in every year for a new chain and cassette for his mountain bike. He had a rather nice (for the time) Marin with an XT 18 speed driveline, but he only used one gear, ever. He rode the bike heavily in that one gear, wearing down the cassette on one cog. It was odd to see 5 brand-new looking cogs, and one which was worn down to the point that the chain skipped over it when he stood on the pedals.