This is one area where it can be very easy to overwhelm a child with autism. For my children, they have a great deal of difficulty processing more than one thing at a time, particularly if they're excited or stressed.
As a neurotypical adult, I'm used to chaining things together. Have to run an errand? I'll stop at two or three stores to be efficient. Spot a library book under the couch? I'll grab it immediately lest I forget it.
But switching gears mid-moment is not one of my kids' strong suits and isn't for many people with autism.
There are two situations which often result in abrupt changes: the just-one-more-thing phenomena and a dispute between the adults in charge over what should happen.
Just-one-more-thing is very seductive and easy to fall into. I said it's time to go to the park but oops, I forgot about doing a bathroom break first. To me, no big deal, we'll go to the bathroom and then go to the park and everybody wins. To them, huge big deal: I was going to the park but now Mommy has said no. And really, they stop listening after that.
I've had to learn to take a moment and go through things in my head before saying anything out loud. It lets me get it right the first time: First, bathroom, then go to the park. If I do end up forgetting something, I ask myself how crucial it is to do right this second. If it can wait at all until the task/activity is done, then it waits, even if it would be more convenient for me to do it right away.
Disputes between the adults in charge is slightly more difficult to handle. And although you may be picturing a shouting-match kind of fight, it doesn't have to be. Let's say I've told my son to take his dishes and put them on the counter. My husband wants him to put them in the sink. Boom! That is a change in expectation from what he was first told and can lead to a meltdown. Even dropping a requirement can trigger tantrums, especially if Alex is already upset. "Put away your dishes" - "No, you don't have to" switches expectations just as much, particularly if the adults then get into a dispute.
It's difficult (particularly for me and my inner control freak) but I have to let him do it "wrong" rather than try to correct him in the moment. The time to correct the situation is either before the instruction is given or after the task is done, when we're setting things up for the next time.