The first indicator that it might be super cheap was the fact that it would transmit on two frequencies; if I recall, those were 88.5MHz and 106.2MHz. "That's strange", I thought, "most North American FM radios won't tune an even-numbered frequency (the .2)."
Once I got inside, I realized the magic. The FM transmitter section was based around a 17.7MHz crystal. 88.5MHz was the 5th harmonic, and 106.2MHz was the 6th harmonic. There was a single transistor that was used as part of the oscillator. "OK", I thought, "that explains the bad sound quality."
Then I started looking closer at the PCB and discovered a few parts that were missing. The scariest was in the USB power section: instead of a 5V regulator (as indicated on the board), there was instead a jumper directly from input to output. That is, it was pumping the noisy, spiky, 13.8V from the car's ignition system directly into the USB port. Luckily, I didn't ever actually plug anything into that port, but it could have gone miserably bad...