I experienced an odd failure mode on my brand-new Thumby after getting it yesterday. While playing a game that uses the greyscale display library (MineSweep from the arcade), a black bar appeared and began rolling vertically up the screen (like if you film a CRT). The bar was maybe four pixels tall and swept up the screen a couple times per second, though the speed changed over time as I interacted with the game. I assumed this was just a timing bug in the greyscale library's display control logic, so I didn't think much of it.
Then, after turning the device off and back on, it entered what appeared to be a boot loop -- the TinyCircuits splash screen appeared for a second, then the display went black for a second, then the splash screen appeared again, etc. But after leaving the device turned off for a few minutes, it turned on properly, and has worked so far since, including in MineSweep.
One final note: before this occurred, I had set the device right next to a cold window (20 F outside). No idea if this could affect operation.
If this happens again I'll record video.