What if Labour somehow managed to get re-elected in 2010 and Gordon Brown stayed on as Prime Minister. The SNP still manage to get a majority in the 2011 Scottish election (due to Labour's unpopularity, bad economy) so the independence referendum is still held in 2014. Then, by a very narrow margin, Scotland votes yes to independence.
Could Brown continue as Prime Minister (up until the general election in 2015), despite the fact that he is Scottish and so would now be a foreigner? His constituency is in Scotland, and all the Scottish constituencies would no longer have MPs in Westminster.
The only way he could continue as Prime Minister imo would be if he chose not to become a Scottish citizen, remained a British one, and then made himself a peer so he could rule as Lord Brown (since he is no longer an MP). But having a peer as Prime Minister would be very controversial in the modern age. Or would there be another option?