I saw that it was happening by checking the file header. The third byte should be a 2 when it's not being compressed, and a 10 when it is. I was using an 8x8 image, if that's significant. EDIT: I can't upload sample images because .tga files are not accepted.
Bug is very simple, just see title. Easy to reproduce, too: just make any image and try to save it as a targa file.