Howdy y'all,
I was looking forward to maidening my new ZMR250 today, but as I was putting the finishing touches on it, (wrapping ESCs in heatshrink, zip ties on cables, etc) I decided to plug it in just to make sure it all worked before putting some props on it and taking it outside. However, when I plugged the battery in at first, the FC (CC3D) and receiver both turned on, but I had no power to any of the motors for whatever reason. I unplugged, checked all the connections, and turned it back on. As soon as I gave it any throttle, there was a loud POP from the front left ESC. I quickly unplugged everything and cut open the heatshrink to look at it. I'm not very knowledgable about ESCs, but the plastic casing over one of the modules on the board had a jagged hole in it, and the heatshrink was charred on the underside. It sounded like a firecracker! As far as I saw, there wasn't any smoke, but I did have a fan going in the workshop so it might have been blown away. I'm using 20A Fire Red ESCs.
What in the world happened? These ESCs were recycled from another, old quad, but they always worked flawlessly on it. I ordered another one because I want to get some air time!

Reverse Polarity?
solder accidentally shorted the components on the PCB?

anyway feel sorry for your problem!

be careful when you hook your new esc up, you might want to check the motor that the esc was connected to. I've had situations where the motor went bad and caused the esc to fail, only to replace the esc and having it fail again due a bad motor, I've also had bad esc's cause my motors to smoke and  frying my motors. so it's always a good practice to check both motor and esc.