I have two Attiny13A-PU circuits on a breadboard. I was having a problem that looked like random resets. I pared the code and the hardware down until they were both running nothing more than a fancy LED blinker, and the glitches were still happening.
I diagnosed the problem and found that they weren't resets, but instead the MCU program counter was jumping to a random location at random times, eventually wrapping around to PC=0 and hitting the reset vector.
Both Attiny's were doing this. I decided to remove one from the breadboard. Problem went away.
I put it back in the breadboard, but 15 rows away. Problem reappeared.
I put it in way at the other end of the 63-row breadboard. Problem went away.
I put the second one back in right next to the first and added more decoupling--I added a 3.3uF in parallel with the 0.1uF that was already there. (I don't have a ferrite bead recommended by AVR042.) Problem went away.
HERE'S THE VOODOO PART:
I pulled the 3.3uF out of the circuit while it was running. Problem did not return.
I've repro'd this three times: Power the circuit up without the 3.3uF, glitches occur. Add the 3.3uF while running, glitches go away. Remove the 3.3uF while running, glitches do not return, for as long as I've watched it. Completely reproducible.
Any explanation? Could this be an electrolytic-homeopathic capacitor?