There's something about screwing the lid back on to a computer's case that seems to act as a magnet for bad luck. Especially if the same machine has been running just fine for weeks or months with the lid off; you finally decide that's cause to declare it working, so you put the lid back on and do up the screws … and the tightening of the final screw, symbolic of your overconfident belief that the job is at last done, is the cue for every intermittent fault, every surprising and hitherto unmanifested bug, every glitch and gremlin and gribbly to come out of the woodwork and make itself known.

Just occasionally there's a causally valid reason for this – the machine actually won't run with the lid on because its CPU fan is dead, or the act of putting the lid back on disturbed a loose connection that would have preferred to be left alone – but usually, it seems to me, it's just Murphy's Law.

(Preliminary indications suggest that the one of these that happened to me last night was not even a fault in the computer in question, but I'm half expecting further nasty surprises…)