AVR-Chat

Another useful tidbit, is to change it around a little, and have it pulse N times in ISR-X and N+1 times in ISR-Y. This way, you can see exactly which ISR is firing. 

Also, as you gain confidence in the code, take out the pulses on the active ISRs, but leave the pulses in for the inactive vectors. Now, if you ever have an ISR active that shouldn't be, you'll start getting pulses. 

The one pin pulse trick can really help.

I did a routine called "pong" that outputs a byte to a single pin, with wide pulses for 1, and narrow for 0, but the byte is always the same width. I've gone as far as thirty-odd bytes this way, since I have an excellent DSO and logic analyzer to read the pulses with.