It was one of those pop-out upside-down 5pin locks on my apartment window that I had lost the key to. Its taken me almost a week to crack ... but the satisfaction of hearing it finally pop open was fairly amazing. Several other locks around the house fell in quick succession afterwards.
It took so long because I was initially trying with paperclips (modified umpteen times), mini screwdrivers, compass tips etc. Finally did it with wiperblade insert torque wrench and a probe in an old high school dissection kit bent into the half diamond.
Thanks to this site for showing me basic picking.
I can see how better tools would have made this much easier.