The single odious Labour candidate nobody likes finds themselves without a seat.
This isn't strictly true as the party election material ranks their candidates in order they want them returned (usually in order of seniority) and urge voters to vote in that order. This way the party's chosen #1 will get returned in the first round of counting and the votes they got over the threshold (number of total votes cast divided by number of seats plus 1) will get transferred to the #2 and #3 candidates in subsequent counts. There's a real science to working out how your voters should vote in different wards to maximise the chances of all your candidates getting returned in the earliest round of counting possible, and voters going against this could really fuck up the party's chances as whole.
Say for example you dislike Labour's #1 candidate in your constituency, so you and a significant number of people in your ward put him as your third choice. Now there's a risk of none of the 3 candidates having enough votes to be elected in the first count and they all have to wait til subsequent counts for transfers from other parties/the Labour candidate with the least votes being eliminated, by which time 3 out of the 5 seats could be taken by other candidates, not 2 out of 5 as planned.