It is usual by this stage in our Scratch adventures that many of the children's scripts have become quite long. Often bits are repeated. The idea that we can tell the computer to repeat a set of instructions as many times as we like is very powerful, but I find children first appreciate it because it means less work!
It doesn't work but that's okay. We'll try again. We'll persevere.
So why doesn't it work? If no one volunteers an answer (someone usually does) I ask them how long they think this script would take to run through. how long does it take for the computer to change a costume. Try it out if they can't say.
It doesn't take long for someone to suggest that the computer needs to wait a bit before changing costumes.
It's okay to make mistakes. It's okay to try something out and then go back and make it work or work better. Children love doing this. And I find it's how they learn best.
I then model how to wrap a forever loop around the bit that was repeated. I ask them what they think it will do. And sure enough it does what they think.
Then I'll show them a repeat block. I ask them what this will do and what will happen if I change the number.
"Can I make it move and flap its wings at the same time?"
I tell them to give it a try.
After all, if it goes wrong the first time they can try again...