But what if you realize, after the encounter begins, that you (the DM) inadvertently conveyed incorrect information to the party or otherwise made a major mistake on the actual encounter? That the group would have acted very differently if they had been conveyed the correct information.
Such that, and encounter you telegraphed as, and intended to be, easy (and that the group engaged on based on the information provided) is actually deadly (for example)?
If you saw the mistake only after the group engaged, would you change anything?