We've all been there, I suspect. You know, the moment after a bit of distraction when you suddenly realise you are just getting to the slip road for the junction you mean to leave at? "Mirror, Signal., Manoevre" is the correct procedure and, if you can't do so safely then you should just carry on to the next junction and either back-track or find another route.
Anyway, any trucker will tell you that far too many cars, so busy on getting past you, leave it to the last minute. Presumably they miss all the signs for the previous mile or decide they have more time to carve across your bows than they actually have? A couple of weeks ago I saw my most extreme case however. I was heading south on M1, just past junction 24 heading almost past J23a when a sports car decided to go the for the 'last minute cutting-in'. Its a very acute angle containing first of all the white painted cross-hatching on the tarmac, then that 'grey area' where all the grit and rubbish collects, then the grassy bit before the armco crash barrier... So the sports car manages to bounce over the grass, just missing the armco, and rejoins the lane he should have been in 200m before. Fortunately there was nothing else already in that space otherwise they would have hit each other.
Totally Without ATtention?
Or, more simply, TWAT (As John Cooper Clark might say)