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Let's say you had a doctoral student who you were main supervisor for {~advisor) throughout their data collection and most of their write-up, but just before they finished they changed to another main supervisor and you became deputy supervisor (you moved to a new institution in another country and they wanted someone close at hand). Let's say you then counted this student on your CV as supervised to completion as main supervisor because this is 90% true and because these CV points are important for things like promotion. The plan if anyone ever called you out on this would be to just explain the situation as it is and say that 90% of the work felt like it justified the claim. The student is in Sweden (where the regulations say students have the right to supervisor change without even having to make a case) and the supervisor is a Lecturer (~associate prof) at a UK university who wants to make Reader (~full prof) some day. Is this a bad idea? What is likely to go wrong?

The fact that you ask, makes me think that you believe it would be at least a bit wrong. Anything that looks like dishonesty in an academic's statements or record can be very serious.

I suggest two things. One is to be completely honest about it as you have been here. But the other is to talk to the student in question and perhaps get a statement/recommendation from him/her about your helpfulness. A recommendation from a student for a professor might be a very powerful statement to a hiring committee.