A coworker made me a USB power injector for my Nokia 770 (I'm useless with a soldering iron). Here it is in all its glory, next to the tiny USB keyboard I plan to use it with:

Nokia 770, tiny USB keyboard, USB power injector

Five days later I discover that it doesn't work any more. Actually, it works, but the battery is dead. I left the battery connected to the voltage regulator, and that was enough to drain it.

Wikipedia says that carbon-zinc 9V batteries have a typical capacity of 400 mAh. ThoughtFix measured the circuit power readings at 4.62 mA with both USB ports disconnected. That's enough to drain a 400 mAh battery in 86 hours or 3.6 days. And here I thought it would last for months...

Time to buy a new battery. And this time I'll keep it disconnected when I'm not using it.