Banking on a single error

The next game i finished was an interesting and long struggle. Why? Because it had only one serious error in 50 moves! There are definitely some inaccuracies along the way, but i don't have the skill yet to fully exploit those. This one error proved decisive, but it came late in the middlegame and my opponent provided tough resistance till the end. I hope that all commentary is instructive and helpful to others, so feel write to write something.

Everyone who finds more errors wins a virtual cookie  

Who made this one error? Gotta see for yourself: