My old loft had a steel doorway that I used for pull-ups for two years straight... it taught me more about core engagement than any stable bar ever did. rose_martin22 mentioned the limited range, but that forced me to control the eccentric phase like nothing else. Actually improved my bar pull-ups because I had to stabilize every millimeter... the instability wasn't a bug, it was the feature. Felt way more prepared for outdoor rigs with uneven bars.

What about epoxy resins? They boast a fast set time, but the full cure often needs a full day. I once tried to sand a river table after just six hours because the surface felt solid, but the heat from sanding reactivated the uncured glue and made a gummy disaster. In my experience, you have to strictly follow the full cure timeline, not just the initial set, or you'll ruin the project. Your mileage may vary with different brands, but I've learned to treat that waiting period as non-negotiable. Now I just set a reminder and avoid the workspace entirely until the next day.