One recent one for me was a PIC which was spontaneously resetting for no 
apparent reason, even though everything I measured looked ok.  It took 
me forever to figure out that I discovered that I had missed disabling 
LVP, and the floating RB4 pin (which wasn't yet connected to anything) 
was sticking the PIC into programming mode as it toggled due to 
capacitive and other effects.

-forrest

Picbits Sales wrote:
> We've all done something similar I would have thought.
>
> I had one last week where the code would simulate fine on MPLAB but when 
> burned to an 18F1320 all I could get out of most of the pins would be a 
> logic high rather than the pulses I was expecting.
>
> Only took me an hour to work out that I hadn't bridged the grounds on the 
> breadboard to the lower half of the board with the PIC on it.
>
> Thank goodness for scopes ..........
>   

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