The first thing I did after opening the box was to probe the 3.3V rail with the BP internal power supply enabled.
Here’s what I saw:
That looks familiar! Except this time it’s an oscillation at 5.9kHz and 400mVpp! (almost 4 times what I saw on the SFE Bus Pirate).
I replaced C11 with a 4.7uF tantalum surface mount cap that I scavenged from another board, and the oscillation went away completely:
Ahh, that’s better!
Again, I strongly suspect the low ESR of the ceramic cap is to blame. The 3.3V regulator really needs to see the higher ESR of a tantalum or electrolytic cap for stability.
My advice to all Bus Pirate owners is to check your 3.3V rail with a scope, if possible, and replace C11 with a tantalum or electrolytic cap. Alternatively you may be able to add a small amount of resistance in series with the ceramic cap that comes with the Bus Pirate, but I haven’t verified this.