Warp Speed Catapult
I was playing orcs in one of the tutorial missions yesterday when I encountered this bug. One of my catapults had a run-in with a few ogres and ended up with very low (barely-visible) health. Since most of my units had been heavily damaged in that fight, I had them all pile up and used my goblins and hero to heal them back to full.
After healing, the catapult that had been seriously damaged earlier started gaining speed stats very quickly. When I next moved my catapults is when I finally noticed it happening, and by that point the one catapult had a speed of over 20k. After asking in chat, I restarted my game at a moderator's suggestion, and the catapult went back to normal.
If I had to hazard a guess, I think this has something to do with the speed of siege units changing with their health, but I haven't been able to recreate the circumstances that led to this bug. If it matters, I don't have any siege unit-related upgrades, and at the time I didn't even have catapults researched. One was a quest gift and the other was a mercenary.