Clive D. W. Feather wrote:

[Warning: vague memories ahead] The original version of estoppel was a
rent case. For some years the landlord of a property accepted a lower
rent from the tenant than was actually due, signing the rent book each
week or banking the cheque or whatever. At a later date he then tried to
claim the difference back. Lord Denning said that he was unable to
because he'd left the tenant with the impression that the rent had been
paid and accepted.

FWIW (not much), I accidentally paid too much rent (maybe about £1 a
month?) for about two years without noticing.

When I moved out, the landlord returned all of the accumulated extra £1s
along with my deposit. (I still hadn't noticed at this point.)

They didn't bother to tell me while I was doing it, mind you! :-)