I managed to destroy a perfectly good carbon seatpost with it. It had seized up w/ its shim in the seat tube and we shot some down the tube. Well lo and behold, I could never get the post to stay in one position, it would always slide down. I ended up cranking it down so tight I cracked the post.

I don't use it anywhere. I'd heard at one point that it was flammable. Besides, when it dries, that residue it leaves behind becomes a dirt magnet.

I read somewhere about using steel wool to get rust off of cables, don't know what grade (have never used steel wool for anything but was looking at it in a store yesterday and noticed it came in several varieties (from fine to coarse).

Some people seem to think they're tribologists. ;)

I routinely clean my chain with WD-40 and a rag, then lube with oil (Tri-Flow is my preferred one, but I'll use just about anything I've got handy, from light machine oil to Vactra #2). If nothing else is handy, WD-40 is better than nothing, but only just - it's too light to handle extreme pressures well, it tends to fly off too easily, and what is left tends to attract dirt. It'll do to get you to the shop, where you should buy some Tri-Flow or similar.

For a really clean geartrain, I'll clean the chain with an organic solvent. Same for the cassette, but be sure to remove it first - no need to de-lubricate your freehub. When everything's shiny and looks brand new, lube with your preferred chain oil.

Check your chain regularly for stretch and replace it only when needed. Otherwise you're just throwing money away.

Once when I was much much younger, my Zippo was out of fluid.

Unhindered by the process of rational thought, I decided to spray a little highly flammable WD40 onto the wick in order to get just one more light. So I dropped the tiniest drop onto the wick and BOOM! - it lubricated the striker wheel and couldn't make so much as an eensy spark. A couple shots of carburetor cleaner and a new flint put it right later on.

As for legitimate uses, I've used it to dry out the distributor cap contacts on an qancient Mercedes to help a young lovely get home from the car wash.

About 15 years ago, a coworker of mine tried to clean a rather expensive adding machine with WD40. It cleaned the machine just great, but of course, it didn't work afterwards.