One of the things you're meant to look out for is air bubbles. Well apparently I did not notice a big one in my insulin reservoir - because for the last few hours I've been getting no insulin. I'm up to ~15mmol/L... and carefully examined the tubing and saw some difference in the light, but I couldn't tell which was insulin and which was air - given I was getting no insulin, I decided to do the "shower/exercise" thing, where you disconnect the pump for a bit and reconnect it again when you're ready.

I suspended the pump, disconnected it, then resumed it and forced through what's called a fixed prime - no insulin dripped out. I did the fixed prime three more times and saw the insulin progress up through the tubing until a tiny droplet formed. I hooked it back up again and gave myself 4 units of correction insulin immediately (because the bolus wizard already thought it'd done the correction to fix my high).

That's definitely an interesting gotcha right there.... sometime during the night it would have finally gotten back to insulin I guess, but I imagine it would have taken quite a while and my BGL would have gone pretty high without any insulin.