I've only been playing for a couple months, so I'm pretty prone to facepalm moments. I just had two recently that ring a bell.
1. Embarked on an area that was too cold, water source froze over. I got this bright idea that if I have a dwarf channel ice, surely that will break it away and water will come flowing out, sounds reasonable right? Well.. I was wrong. Not only did it not accomplish anything, a dwarf became encased in ice, never to be seen again.
2. Had a few dwarves channeling an area from above ground to reach a muddy spot for an underground farm. It worked, sure. One problem was, I didn't have a way for the dwarves to get back to the surface. I had the upward stairway, but the problem was that there was ground in the way. They got down there because it was from a slope, and there was no way for them to get back up. I thought it was weird.. dwarves standing around, but for some odd reason it never clicked that they were stranded there. I only really understood this when a dwarf starved to death. <-- And that's actually my current fortress, not the ice incident though.. that was a previous failed fortress. The main reason I didn't notice the dwarves stranded was that I was very busy around my fort. This was only one of my projects I was working on.
If I do anything else extremely facepalm worthy I'll be sure to let you all know.