It's traditional for writers to think little of actors. It certainly can be frustrating to watch some actors 'not get' your masterpiece, butcher the lines and miss the jokes.
This, of course, makes the assumption that your script if perfect, which it isn't. And that it's funny, which it might not be. And it's also easy to forget that studio sitcom, at least, is really hard on the actors. An actor has less than a week to rehearse half an hour of comedy that will be shot in front of an audience and then broadcast live to millions of people. Half of that rehearsal time will probably be in a church hall somewhere in West London, with tape on the ground showing where marks are. The script changes overnight - and there's no certainty of success. At least with The Merchant of Venice, if it isn't working, the problem is not with the writing.
At the first read-through, the actor is probably sight-reading. They may not have been sent it in advance. They rarely read it in advance once the show is up and running, but that's forgivable. This means that some lines get fluffed. Not as many as you'd think. But an innocuous-looking line that's been pored over for months by the writers suddenly becomes hard to say. Usually, though, it's a mouthful line that's proving to be a problem. Often the second sentence in a speech. Occasionally the third (should you ever stoop to someone speaking for that long uninterrupted).
It doesn't happen in every readthrough, but now and then. The actor fluffs a line. They make a 'bleurgh' mouthful noise. Apologise. Everyone laughs. They do the line again. Fluff it again. Then again, although read wrong, but we're over the worst of it and the scene continues.
I have no idea if anyone else does this, but my policy is normally to change the line. Don't make a big thing of it at the time - or put your hand up and say 'My bad. Chunky line. We'll fix that' and then in the dead of night, change the line. It may be your favourite joke. It may be very easy to say in your mind and with your mouth. You could have been actor if you'd wanted to be. But you chose to be a pasty-faced writer. So get over it.
The actor might get the line right the next day. But probably not. People will be wondering if they'll fluff it again, especially on the night. They'll get nervous. My advice. Lance the boil. Change the line. It's just a line. Think of a new one. If this is Draft 5, most of the other lines aren't original from the first draft, so why be so precious over one? If you huff and puff and demand (privately to the director or producer) that the actor 'get over it' and 'get it right' because 'this really isn't that hard', you will be the loser in this. And let's face it. You're a writer. You're already a big enough loser.
Bits of advice like this will abound at the comedy writing course I'm running with Dave Cohen in London on April 20th and May 4th 2012. More info here.