I am sure there are lots of things that we have all learned the hard way. I know I have learned the hard way a lot over the years and even more the last 6 or 8 months.
The latest was using little brass screws.
You have to pre-drill as deep as the screw in hardwood and you better use bees wax on them and then you don't tighten them like they were a 5/16th bolt or.... they snap right off.
On the two boxes I am building right now it took me 3 screws before I got it though my head, had to drill out 3 screws then plug the holes and redo them.
So what is one thing that you learned the hard way?