When trying to play youtube videos in Totem, I get this error:
GStreamer encountered a general supporting library error.
When starting from the commandline, the actual error is is this:
Message Error: GStreamer encountered a general supporting library error. gstffmpegdemux.c(1255):
gst_ffmpegdemux_open ():
/GstPlayBin2:play/GstURIDecodeBin:uridecodebin1/GstDecodeBin2:decodebin21/ffdemux_swf:ffdemux_swf1: "Input/output error"
There are several threads floating around other sites for this problem in 9.xx, but I'm on 10.10, and supposing the proposed updates are long propagated.
I have checked and the Gstreamer FFMPEG plugin is installed: everything is basically stock.
As Törzsmókus pointed out, this is a known recurring bug. Totem's Youtube interoperation is achieved by reverse-engineering the Youtube API. So whenever Google changes the API, Totem breaks. The bug is marked fixed, but if it reappears, file a bug or comment on Launchpad.