#1 really got to me. I thought I was going to drill four holes with one bit. Honda Fit, in my case. If I knew that it was going to take me all day to drill four holes, I would have paid a shop to do the install.
#5 got me. The cuts the instructions said to make turned out to be unnecessary, a different piece needed to be cut out.
My installation required a remove and replace of the bumper core, to get at the installation location behind the core. Turned out for this installation the core is slightly sprung when installed, so it would have helped a lot to have a second pair of hands for a pry bar while lining up bolts to reinstall that piece.