So here we are again... Structure. Oh, come on. You must have known THIS was coming (oo er). I've already written about fatty dialogue, don't care characters, murkiness and abrupt genre/tone change. It was only a matter of time! ; )
There are many ways bad structure can screw up a script. I've covered most of them on here, but the one I see time and time again is meandering structure. In other words, characters do one thing... Or another thing... Or another... For seemingly no particular reason, at least at first; sometimes for no apparent reason at all. This means that often it's hard to invest in the protagonist's journey. Great phrase that, isn't it? Very script readery. Let me spell it out what it means in plain English:
You have a mate and s/he comes round a lot for a cup of tea. You haven't seen them for a while and you're pleased to see them at first... But slowly and surely things change. They keep harping on about things that have happened in their life, but they don't bother to really clarify their story - they reference people you don't know and ignore your questions as they listen to the sound of their own voice. They don't set up things and they don't pay them off. They talk about one event... Then another... Then another... With no apparent connection. Before long you're bored - you don't know what's going on, because they haven't involved you in what they're saying. You end up tuning out, despite your best efforts to make sense of it. See? You can't invest in their JOURNEY.
Often meandering structure will come hand in hand with a passive protagonist: in other words, a protagonist that events happen TO. Now don't get me wrong: sometimes a protagonist won't be in direct charge of what happens to them, especially in comedy - and there IS more than one way to skin a cat or write a script. What's wrong with having a secondary character pull the strings of your protagoinist? But this can only go so far: at some point your protagonist is going to have to step up and do something. Even a passive protagonist has to make some sort of decision, have some sort of idea of what they're doing - else why are we watching them? What is interesting about them? What is it about their lives that makes us want to - you guessed it - invest in their journey?
Journeys have starts, they have middles, they have ends. They have to, just as they have someone kicking it all off - whether it is your protagonist or your antagonist. Your screenplay is the same, whether you believe in The Three Acts, The 22 Steps, The Min Movie Method or anything else you care to mention. The difference is how you play it - and you must decide HOW to play it. Else your script will meander.
But end of the day, who says a writer *has* to do this or that or the other on pain of death?! Not me. You can of course do whatever you want, The Script Police are not exactly going to knock down your door - but it is handy to know the difference between changing or challenging the so-called rules and making a classic script mistake.
That's not to say of course they won't happen anyway by the time you get to production... Any worst produced cases in your view of these five mistakes? Over to you....