Trying to give you as much info as possible for troubleshooting purposes. Here's what happened, what I've tried, and where I'm at:

So for Halloween I put on a nice little light show for the trick or treaters. I had a few too many lights hooked up because after about 10 minutes, breaker 7B tripped (15A breaker = 120V @ 1800W). This happened probably 3 times. The only reason I allowed it to keep tripping was because there was so much time (5-7 minutes) between the trips. I figured if it was over the limit, it would pop immediately. So eventually, I removed a light (300W to be exact), and a few minutes later it tripped again. I removed another 300W and all was well for the rest of the night. Except...

Breaker 7B is of course adjacent to breaker 7A. They're both 15A. It's a combo breaker. I had a 1500W load plugged into 7A (lights and a fog machine), but it never tripped. Near the end of the night circuit 7A died. Breaker 7A didn't trip. 7A is my living room. I ran an ext. cord into the garage from the living room to power the additional lights. My 3-prong circuit tester tells me the ground & hot are reversed. I know this is likely just an open neutral so I started looking for that. I rewired 4 outlets, cutting the wires out of the holes, and restripping them and wiring them to the screws. They are daisy-chained.

Now with my 3-prong tester in the circuit, with all other loads removed, and the breaker on, I have one bright amber indicator, and one dim amber indicator. If they were both bright, all would be perfect. When I flip a light fixture on in the circuit, the bright one stays bright, the dim one goes out, and the RED one comes on bright (indicating the open neutral).

I've checked the panel with the cover off. All screws seem tight. Do I have an open neutral? If so, how do I find it? Is it a bad breaker? If so, I'll replace it tomorrow. Thanks for the help.

You need a voltage tester to confirm whether a circuit breaker is defective, or if you have an open neutral.

I solved this one myself the next day. The breaker was fine. I measured the voltage going into it, and the voltage coming out of it. The breaker was doing its job. There was an outlet I missed. It was hiding under a table. It was the outlet nearest the light switch (which was also on the same circuit). Removing the wires from the push holes on the back of the outlet, and wiring hot and neutral to their respective side screws fixed it. It was indeed an open neutral thanks to those silly, weak push hole terminals on the back of the outlet. My advice is to always use the side screws.