Just got done modifying a 10 ton fertilizer cart for a big vegetable farmer in our area.  We have done a lot of work with this operation over the past few years.  The job was to change the tread width from 80" to 60" in order to fit down the rows of the new planter, and make a new telescoping tongue 110" long.  Everything went great, it went straight at 45 mph, so at planting speeds of 3.5 mph, it was surly going to go good.  He took it and tested it, and everything was great, but he wanted a few supports added to the tongue.  OK, no problem, and 2 hours of work........That is until I was done, and lifted the tongue and it only moved up 3 inches before it bound up.  Hummmmmm, that's odd I thought.

Well in my infinite wisdom, I added the supports, but the pivot point of them was 2 inches behind the pivot point of the tongue.  So you can imagine how well that worked, a pivot point with 2 different axis of rotation.
In our constant quest for quality work and customer satisfaction, I knew there would be no "easy" way to fix this.  I was forced to cut off everything I mounted, and re-do it all the right way.  So now my easy 2 hour job was quickly turned into a 7 hour ordeal, not one of which was billable to the customer. 
That's a good way to make a good job turn into bad job in a hurry!!  "What an idiot I am" is all I could say to my wife when she asked why I needed to work in the shop on Sunday.....for free.