I got a SainSmart digital oscilloscope kit for christmas, and put it together according to documentation I found on the Amazon.com listing for the item.
The markings on the board directed me to connect the input pin and output pin opposite of how they should have actually been connected. That is, it was receiving 9v to the output pin instead of the input pin, and the input pin was connected to where the regulated 5v output was supposed to go. I discovered this by nearly burning my fingertip on the overheating voltage regulator after plugging in the 9v power supply, and then looking up the voltage regulator's datasheet and comparing it to the board diagram.
I removed the voltage regulator, turned it around, and soldered it back in place. When I plugged in the power supply again, the regulator got hot again, and I still didn't get the expected 5v at the output. Is my voltage regulator fried?
(Note: it's either a 78L05 or a 79L05 - the board has one of each and I don't recall offhand which one was the one I was dealing with.)