...you're well into a 'groove' of construction, only to be held up by something you don't have.
For me, I was about to mount new modules to the baseplate yesterday, and despite having a LOT of M3, M4 and M5 fastenings, the longest M4 I had available was a 16mm, and I really need a 20mm to get more than a turn into the baseplate!
Ordered 100 and hopefully here tomorrow.
Snapped drill bits are also somewhat deflating.
So, what stuff stops you in your tracks?