I suppose I make the most mistakes when I am following a pattern, which is not that often for me — but almost always happens when working with either complex lace or colourwork, of course. I have never found that the recommended tool of a lifeline does me very much service – but that’s mainly me. So, I typically rip all the way out and start over, or end up freeforming the rest of the project, if it so allows.
It’s been a while since I have made a devastating knitting error thankfully, but the last one I made was this hat and I totally didn’t notice until I took this picture of it. The chart somehow ended up 1 stitch off, but only for one side and then magically corrected itself. In the end, it’s not that big of a deal, but looking at the picture totally annoys me. 🙂