I have observed this several times (full disclosure: most recently happened to me. But I considered it a 1*&k move even prior to that event):
- User observes that a closed question is closed as "unanswerable" incorrectly, and that they know the answer
- User requests the question to be re-opened
- User waits for reopen process to happen since they can't answer yet, but have to leave because we all have lives and sleep outside of SFF.SE
- Meanwhile the question gets re-opened
- Someone else swoops in before the original user arrives back, and answers it with what the original user's answer was supposed to be.
On one hand, this does not violate any of the rules, and doesn't really damage the site in aggregate (good content is generated either way).
On the other hand, this DOES have negative side effects:
- Original user's efforts are NOT rewarded (it took time and effort to write up a "let's reopen" Meta post).
- Feelings are hurt
- The user who suffered this treatment will be extremely unwilling next time to improve the site by spending their effort on reopening the questions that should be. That leads to less resources to maintain the site long term.
What I'd like to propose is etiquette whereas in situation like this, the person who said they know the answer but can't post it yet is offered a grace period (let's say 12 hrs from question reopening) to post their answer without other users swooping in and stealing the thunder.
It wouldn't be a hard and fast rule, just an encouraged behavior norm.