Much, much later on, and you're stuck. The plot can't move on ahead or it's set off in more directions than a catherine wheel. The reason behind your plot block is the problem/career/best friend - they don't fit comfortably with the current situation. So you puzzle away, trying desperately to work out how you can get round the situation but not succeeding.
You have a hangover! Something you randomly chose in the past is now affecting your writing in the present.
I've just been stuck in hangover hell, having made a character married. This meant that further down the plot line he had to have a wife in tow. This wife was causing me problems - it wasn't plausible that she'd turn up all of a sudden, but she existed so neither was it plausible that she wasn't around. Then - ping! Light bulb moment. Just because the character was married in the first draft didn't mean he had to be married in the second, especially as I'd rewritten him pretty much from scratch. The wife could go. End of problem.
Sometimes it's really hard to spot hangovers. You get stuck in a particular way of thinking and it's hard to challenge yourself and ask, Why can't X do Y? Why does Z have to be female? Why can't A do that? Why does B have to be a teacher? Why does C have to be tall? Why can't D be unable to swim?
In A Single to Rome I struggled through the first draft with lots of hangovers and a hefty headache to match. Then I realised that Teresa was doing two plot functions and could become Teresa and another character - Olivia. Judith didn't have to be female, and became Bob (who hooked up with the newly created Olivia). Natalie's original job got the chop and she became a lawyer, which both raised the stakes and solved a couple of plot problems.
So if you get stuck, check that it's not because of some hangover from the past. It may have been the answer to a problem then, but that doesn't mean it still is now. Challenge your previous decisions and check you haven't got a writing hangover.
And if you have, the solution is straightforward. You wrote it: you can delete it. It's as simple as that.