// Has there been any instances of this in the past ? //
Yes. Back in the 90s Labour under Kinnock were odds on to beat John Major, when right at the last moment they issued the killer suicide blow on themselves by deciding to hold a victory celebration before the election had actually taken place.
The sheer arrogance of it, coupled with the disturbing site of Kinnock, Hattersley, Prescott etc disco dancing, high fiving and group hugging prompted all the floating voters to rush out and vote Tory, causing a shock defeat for Kinnock.