I lost my rag for the first time while working on my cab last week. I can just almost laugh about it now for the first time...
Long story short I had to cut out a rectangle, one side with a 45deg angle on it. First 3 sides went perfect and one the final cut my wood moved slightly, moving the guide slightly. The fact that it only moved slightly, combined with the dull atmosphere of 'the shed', combined with steemed up goggles and damn jigsaw blowing sawdust into my face meant that I didn't actually notice - until it was too late.
I spend 2 or 3 minutes considering if I could fix the ruined bit of wood. Then wack across the workmate. Oops now the workmate is broke.
Every cloud has a silver lining tho - I repaired the workmate with the messed up bit of wood (it just happened to be the same width and thickness). Now my workmate is better than ever!

(you can just about see the mistake on the right)