A funny thing happened at Ed Byrne's Riverside gig last week. Well a few funny things actually, because it was a pretty good show. But the funny thing I refer to here m'lud was Byrne coming back onstage to do an encore. After a month of shows in Edinburgh where acts do their allotted hour and then get out sharpish this was a genuine surprise. I had almost forgotten that theatre shows usually include an obligatory add-on at the end.

This did get me thinking though. Do punters really appreciate this tired old convention when they all know it's usually fake and what they are really seeing is the last ten minutes of the scripted act? Byrne jokingly referred to the artifice of the encore while still doing it and then earlier this week at the Bloomsbury Theatre Mark Watson dispensed with any pretence at all. He just said he was finished and was going to get offstage now, except for briefly hiding in the wings before coming back on to take modest bow rather than tell more jokes.

There has been a bit of talk in the blogosphere about the length of rock gigs recently and comedy is in the same artistic ballpark. Sometimes you can have too much of a good thing and when a performer has done, say, 50 minutes of comedy either side of an interval, is an extra 10 minutes really necessary? Comedy is also like theatre and Ken Branagh doesn't do his favourite ten minutes of Ivanov again when he gets a standing ovation.

Yet successful comedy shows feel duty bound to do an encore. Little Britain's live extravanganza concluded with an all-singing, all-dancing costume change return featuring Matt Lucas as only gay in the village Daffyd and David Walliams in a PVC policeman's outfit, complete with shiny hotpants. Hardly the sort of thing that could have been made up on the spot.

On the other hand an encore can sometimes feel like a nice bonus. Like going to Somerfields to buy some Tetley Tea Bags and finding that they are giving away an extra 50% (which curiously enough happened to me at the weekend, oh my glamorous life is one long hedonistic party...). But genuinely spontaneous moments are few and far between.

In fact when I think about encores the funniest ones were the ones that shouldn't even have happened. Such as when the performer - no names, no pack drill - rushed back on after a lukewarm gig to do their totally unplanned encore and had to be quick because the so-called tumultuous applause was already dying away. Best of all though was the gig when a famous comedian - who shall be nameless for the sake of his career – stepped back onstage just as four people in the front row were ostentatiously putting their coats on. Maybe it was time for him to get his coat too...