I don't know if anybody has seen this but its during Somebody to Love about 3 quarters of the way through, he is playing the guitar and he looks like he forgets to go and sing because he suddenly looks surprised and runs over to the mic
To me it looks like john had already spotted that Brian had missed his line and John sang a bit then instead
It might be rubbish but take a look, i can't send a link as work have banned youtube from the computer
Sleeve liner notes will mention how it's the first time CGI has been used (ever!) and how Queen had to build the computers for that purpose from scratch. These were then sold to James Cameron, of course.
For more details, see the 10 minute Making-of documentary available in the bonus disc (which is ALL there is to see in the bonus disc, btw).