(this is pretty basic, I'm sort of writing it to use for something else)
Conflict: A disabled person is faced with some kind of task that because of their disability seems impossible, or, if not completely impossible, so incredibly hard and draining that it probably wouldn't be a good idea to take on. This can happen in two ways:
1. The person is expected to complete the task. Other people don't think of the person as disabled; or, people like family and friends are very intent on the idea that while the person may once have been disabled, they are now not disabled anymore. Because of this pressure, the person doesn't feel like it's acceptable for them to say, "I can't do this." If they refuse to complete the task or fail in the attempt to complete it, they will either be seen as a non-disabled person who is lazy and weak, or they will be recognized as disabled, and demoted to the stigmatized category #2 experiences.
2. The person is expected not to complete the task, or no one wants them to complete the task. However, the completion of the task is the only way for the person to get something important to them; in the eyes of other people, the only thing that will make the person qualified for that important thing is the completion of this particular task. (Tell me if this is too abstract, because I can think of a lot of examples but I don't want to lengthen this unnecessarily.) If the person does not attempt the task, or tries and fails, they will have to continue living the life that's expected of them, which they don't want.
Resolution: A lot of disabled people end up doing things that are very very hard for them. Some people fail. And often success can feel just as bad as failure because working so high above your ability level can have effects on your mental or physical health, relationships, and general quality of life.
The Classic Disability Catch-22
Some of the people who do this may happen to refer to themselves as disabled (or as having whatever their particular disability is). They may do this to explain a problem to someone in their life, they may do it just in the process of describing themselves, or they may be identifying as disabled while they are engaging in some kind of self-advocacy or disability rights work.
And when this identification happens, other people often respond, "You're not really disabled, because you completed this task." (Or they admit that you are disabled but they say that you're not disabled enough to count.) Disabled and non-disabled people both engage in this sometimes against disabled people.
I don't necessarily like the idea of saying that certain phrases are always offensive and shouldn't ever be used. I guess I can imagine there probably is one person in the world claiming that certain things are really hard for them when they really aren't. But I find The Classic Disability Catch-22 to be such an extremely hateful and unfair situation--to basically refuse someone their identity or refuse to listen to their experiences as a disabled person, because of "the task" which is frequently making their life unbearable. It's basically like having people deny you your identity because you smashed your finger in a door. Smashing your finger in a door already sucks, guys!