I was trying to hook up an LCD to my custom AVR (not really Arduino) prototyping board-and couldn't get it to work. I've tried literally everything. I was pretty sure the code is fine and I've even continuity-tested everything.
Turns out I have (some 2 years ago) thoughtfully added a turn-off jumper to my prototyping board so that I can disconnect everything for the lowest possible current drain. My LCD wasn't connected to the power rail. I didn't realize since the backlight worked anyway, powered from the enable pin. OH WELL.
I have a bunch of piezos I bought from Digikey. Standard practice when faced with a few possibly equally-good options (in this case, 2kHz or 4kHz), I bought a few of each to try out and form a preference. Unfortunately, they look EXACTLY alike. No distinguishing marks that I can tell. Naturally, I found this out after dumping them all on the lid of a parts tray while I made room.
OK, no problem -- I'll just test them all to see whether they react better to 2kHz or 4kHz square waves. And hey, why not use a 555 instead of "gilding the lily"(*) with a micro? So I grab a breadboard and start building up an oscillator circuit. But it doesn't work ... which makes me feel a little dumb. I mean c'mon, it's just a 555!
After a while of probing pins and seeing results I did not expect, I finally noticed: Hey, isn't the pin one notch supposed to be on the other end? D'OH! 
In my defense, I will often mount a quad op-amp upside down since the V+ pin is on the bottom. For the sake of my pride, I'll say it has something to do with that...
(* Been watching a lot of EEVblog videos... :.)