Okay, so I have a ton of multimeters — I think around 5. I’m not really sure why; I think almost all of them I’ve gotten for free except for my favorite one, a Sinometer VA18B, which has a ton of functions and can also send data via USB.
But, I left a battery in it and the terminals got so corroded that they were destroyed. 
So I thought I could quickly solder a new connector to the board.
Rule #1 of electronics: DO. NOT. TAKE. SHORTCUTS.
I didn’t want to take the board out of the housing as shown above, so I thought I could just desolder the existing wires and feed the new ones in. I ended up lifting the pads, making those spots that I was supposed to solder completely useless. Furthermore, I accidentally had my iron temperature set too high, so I was melting a lot of the insulation (the high temperature also caused the pads to lift).
So, I fixed the problem by basically tacking the wires onto the sides of components that were electrically close to the original holes. Mechanically, this is a very bad idea; the connections are very brittle. Take a look at this mess:
And to top it off, the replacement 9V connector I had was too big for the plastic slot, so the battery cover didn’t fit on very well at all. Ugh.
So, morals of the story:
- Don’t leave dead batteries in your devices.
- If you’re going to do rework on a board, keep the iron temperature as low as possible.
- Take the time to undo the extra few screws to have better access to the board. Otherwise, it’ll take you more time in the end and you’ll end up with a worse result.
On the plus side, my multimeter does in fact work again, so I’m happy about that. 
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