Nokia and Microsoft
Absolutely, 100%, completely the wrong decision.
They had Symbian with well-designed phones and were doing well. Popular smartphones beginning with the iPhone came out and they failed to react. They released a few rubbish attempts at smartphones still based on Symbian which failed due to having no ecosystem. At the same time they kept Maemo and Meego on the back burner, doing not very much at all, hoping either would stir themselves into life.
Then the iPhone and Android handsets started duking it out, with Android very quickly catching up. Nokia then finds itself in a position of rapidly declining market share with nothing in the pipeline for rescue.
If Nokia had put a good chunk of their R&D into Maemo and really pushed it, it would have been what Android is today. They're both Linux-based mobile OS's but Android has had a lot more R&D from Google refining how it works so now any handset maker can use it to push out a nice smartphone. Meego never really got started.
So what Nokia should have done was announce they were going to be heavy players in the Android ecosystem. They have a massive team of Linux developers who could have pulled Android in their own direction. Code submitted by Nokia would be taken into the Android source tree and could have effectively made it their own OS with their own closed implementations alongside the open-source stuff. Then they would be competing with Motorola, LG, Samsung, HTC etc with the same OS, customised and made to work better by their talented developers, but riding on the still big Nokia name. Nokia could never have completely shut Google out, but they could have used it as a lifeline until they were on the up again and then been in a better position to dictate to Google, in a position to fork Android if they so wished.
It would actually be really easy to make a massively selling smartphone that could have saved Nokia. Look what Samsung did by accident with the Galaxy S.
Instead they go and team up with Microsoft to get an OS no-one wants anyway (apart from a few Microsoft devotees) to save their business. Wow. No matter how cheap licences for WP7 are, they're still not free. Nokia have snatched defeat from the jaws of easy victory here. What utter fucktards.
It's like someone on Twitter said, when you're drowning in the toilet bowl, you'll jump on the first turd you see to avoid going down.