It's rare, but it happens.
Back in the January of 2013, up in the German state of Lower Saxony, they held an election. The CDU held a few points lead, but not enough to have a majority. So, you'd move to the coalition-construction phase. Party number two in the election....the SPD....refused to partner. Party number three....the Greens....refused to partner. Party number four would partner but they didn't give enough vote to settle the situation.
By German law, the situation is handed to the second-place winner....the SPD. They went to the Greens, and a partnership was built. I must note.....a ONE-vote majority then existed. It was a fragile partnership, but for four years, it's held. There's supposed to be an election in January of 2018.
Well....one member of the Greens got peeved with her party. Chief reason? They'd closed off a move over to the EU parliament for next year. In effect, she was finished within the party. So she said fine....she'd quit the party but maintain her seat....and flip to the CDU. At this point, the CDU now holds a majority in the state parliament. Pretty shocking for the SPD and Greens waking up to find this out.
The SPD? They are standing there and are kinda wondering about the future (five months until the election). In some ways, it's an embarrassment as to the way that the Greens handled the way with this Frau Twesten.
The effect on the January election? If you toss in the likely Merkel win, and the Green member shifting to the CDU....I'd take a guess that the January election will be a fairly solid CDU win (more than the 36-percent of 2013. I'd also take a guess that the Greens won't match the 2013 win (13.7-percent) and will fall two or more points short.