On the off chance that some people are a bit thick and don't realise, rather than just purely selfish....
I am using a wheelchair. Like the majority of other, healthy, people I realise I need to use the toilet, run of the mill shopping centre stuff. However, unlike the majority of people, i have to self propel my wheelchair (with bad wrists and hands so I am slow), so it takes longer and it is painful to do. The nearest toilet isn't accessible so I wheel to the next toilet, further away which takes longer. Someone is in this toilet so I wait, sitting outside (and it's NEVER a minute, normally five minimum). When the toilet is free I have to manoeuvre myself through the door, get it closed and locked and then remove my clothing and transfer (I don't feel like being more specific but I'm sure you can imagine how much harder it is). Then do it all in reverse to get out.
So, in this scenario, if the person coming out of the toilet is disabled we normally share a glance of empathy, no harm, we are in the same situation. HOWEVER if the person is not disabled and comes out, (and they ALWAYS do this, always) blushes and says "oh there was a queue for the other one, I didn't think anyone would mind and I was only going to be a minute", quite frankly, I think they are a selfish, inconsiderate arse, and generally tell them so if I don't mind the attention from other people staring.
Bear in mind, I am in pain every single minute, wheeling around and even sitting in the chair so this can be the final straw. It's no wonder I barely leave the house these days, and it's not my disability that causes this, it's other people.
If even one person realises the inconvenience and selfishness of their actions, it will be worth it, but should I have to justify this by detailing how I go to the fucking toilet, really?
So next time someone who isn't disabled uses the accessible toilet, bear in mind you might come out to meet me sitting waiting, with my MN vocabulary and my grumpiness!