OP, I would say I'm surprised you just learned that lesson, but I've had to learn that a few times now.
Today I started work on laying paving stones for a dog kennel, pretty simple right? Except I thought it was 12x12 instead of 10x10, so I bought about 50% more stones than I needed. And I underestimated the weight of the stones so much that I put almost a ton and a half on a cheap 5x8 trailer. Then I cut some shallow ruts through the back field, until I tried reversing into place and the trailer got stuck, then I cut deep ruts as I went back and forth. I then realized my building area was twice as un-level as I thought it was and got a wheelbarrow of dirt from another part of the yard. As I dumped it out, I realized it was actually twice as un-level as the estimate I had just made. Then as I was laying stones I realized they were going to be a real tight fit, when I should have a foot on either side of the pad. By then I'd lightened the trailer enough (down to about 3/4 ton) to back it the rest of the way to the site, at least that part went right. I decided to take my small victory and quit while I was ahead, figure out wtf I was doing and come back tomorrow.