it happens now and then. I vaguely remember it happening to a Labour candidate in Redbridge in 2010 I think who was a teacher, and a couple of Lib Dems elected in the 2011 assembly were declared void initially, but were allowed to stay on because they quit from the bodies that they were part of and because thy had taken advice from a Welsh-language document that had not been updated to match the English one.
I'm pretty sure that Void and re-run is the standard response, the famous case of Tony Benn's conservative runner up getting the seat* was because it was clear to all concerned that Benn was illegible (as he had already been disqualified by inheriting his father's peerage, hence the by-election) whereas the issue at this byelection was only raised (AFAIK) during the count
*the same thing happened to a pair of Shinners in 1955 who had been convicted of Treason Felony, their Unionist opponents were declared the winners as their disqualification was well known (and advertised in the papers by the UUP) although one of the Unionists was found to be illegible and another byelection was held
