My understanding of a mixed session is that it is a mixture of singaround and tune session with joining in encouraged - singers are encouraged to sing in keys so that players can join in, especially with well known songs.

I think the person described above who sang in C# was somewhat discourteous, given the type of session it was supposed to be. Surely he could have sung in either C or D. I find much of the time the key I can sing a song in is not critical. There are many songs where I can quite comfortably go up or down a tone to make it possible/easy for people with instruments to join in.