I'm quite fortunate in that the consultant testing my deployments (pre-release) works in the same office as me, in fact probably only 10-15 metres away. He's working through my latest update at the moment but earlier he found a problem.
We sat in front of my PC and recreated the issue and we both thought this has to be static data. I logged on to the Database (using our custom tool) and yes, that was the case so I quickly set-up the data as it should be. Re-test: fail... uh-oh... so I start digging in to the logs but everything seems fine but it's still not working right. So I whip out the code and start testing the function that seems to be getting the wrong value.
After 30 minutes of frustration I suddenly realise that the data I entered hasn't made it to the server... what's going on... oh no I've been really stupid. I'd updated the Dev database instead of the Testing database!
Fortunately we both had a good laugh about that, it meant that there wasn't a rather major issue in the code update.