WARNING: Requested formats are incompatible for merge and will be merged into mkv

[ghost]

Installed a fresh Ubuntu 15.10 two days ago. Added wepud8's youtube-dl repository as usual and now get this error on every 2nd video from YouTube.
Just purged that version and installed from here. But the same happens.

[dstftw]

It's not an error it's a warning that is caused by opposite video-only and audio-only formats (e.g. webm+m4a) that can only be muxed in mkv.

[ghost]

I'm not requesting to download video & audio separately. Just 'youtube-dl http:.....'
And since that never happened before, it IS an error.

[dstftw]

If you don't want to download separate formats you have to explicitly specify format to be -f best. However you won't get best overall quality most of the time since it only served by separate audio/video formats.

[ghost]

That was NEVER before necessary, it always downloaded the 720p version without ANY trouble..

[dstftw]

Default behaviour has changed a year ago thus you had an outdated version all that time or had no ffmpeg installed.

[ghost]

First is true, the latter not. But then, the old default was much better.

[ghost]

Though, my youtube-dl version was up to date. Only old config files perhaps. But I never created any myself.

[dstftw]

I've already suggested how to enable the old behavior. The one of the youtube-l's main aim is to download best available quality media by default that is only possible with separate formats. If it does not fit your needs it does not mean this behavior is worse.