At one time a major US airline had a rule that stewardesses may not marry!
I learned about that while I was Grievance Officer for our local teachers’ union.
Preparing to argue a grievance against our school board in arbitration, I conferred with our lawyer who suggested the tactic of estoppel.
He said his firm had represented the stewardesses of a major airline where many had been fired for violating the no-marriage rule. At one point their union reps had come away from a bargaining session with the understanding that the airline had agreed to rescind the rule and they posted a notice of that on bulletin boards on appropriate airline property.
Happy stewardesses promptly started wearing their wedding rings to work and were promptly fired.
The union objected, arguing that the airline had to know what the union’s understanding was and so was estopped from the firings.
A funny thing: I and the board had agreed to save money by substituting a sound recording for a court reporter so that a typist would have to work from the tape. When she came to the unfamiliar term estoppel, she wrote gestapo! That amused everyone, and I didn’t bother to have the transcript corrected.