When I finished threading my warp, I discovered I had some unexpected threads left over. Oops. So when I sleyed the reed, I painstakingly double-checked every threading and caught a few mistakes.
In one case I had to insert temporary heddles, and halfway through I found I threaded a pattern twice in a row, so I had to shift the remaining 200+ threads, a pair of heddles at a time. After 13 hours total to measure warp and dress the loom, I'm now ready to weave.
