Any examples of this actually happening?
Third Party votes have cost Democrats elections. Most famously, Ralph Nader in 2000 and Jill Stein in 2016. I can't think of any times when Libertarian votes have been a deciding factor.
Libertarian is just a conservative bereft of any common sense.
Janet Napolitano for starters. Many others. Half of the time the Libertarian is a rat operative running to bleed Republican votes.