When a user is banned from asking questions, they are still allowed to ask one question every six months, in order to try and get their ban lifted in case they're unable to improve their existing questions.
However, when this 6-month limit opens up and the user proceeds to type a question, there's no indication anywhere that they are banned or that their ban will continue after they post that question. The form literally looks the same as it does to non-banned users.
Often, I've seen cases where users post a bunch of bad questions and get banned, and forget about the ban. Then, after a long period of time (in most cases, several years), they have another question they want to ask, so they go and post it. When they type up and post their question, at no point are they told about their prior history and that they were banned in the past due to bad questions. Then, when they try to ask another question, they all of a sudden find that they are banned without any advance warning.
In many of the "why am I banned" questions that come up here and on Meta Stack Overflow, this is the cause: they were banned due to bad participation years ago, and the previous ban was immediately re-imposed after they asked one question. As they'd forgotten about the previous ban, such users are left wondering why they were suddenly banned for just one question which may not have been negatively received.
An important point: this is not a request to change the ban criteria. I'm in favor of giving more weight to the new question as opposed to the current model of simply considering all prior contributions equally with the one new question, but that's outside the scope of this request.
Following a suggestion from a comment on Meta.SO, I'm requesting here that a warning be implemented much like the following:
You are currently banned from asking questions. However, as it's been more than six months since you asked your last question, you are currently allowed to ask one question. Please note that unless this question is received positively, your ban will still remain as is.
Can we please have this warning before a banned user whose 6-month limit has opened up asks a question? This also has another advantage: it will remind such users of the quality rules and make it more likely that the question will be high-quality.
