There once was a Rosh Yeshivawho said on Purim that they can ask him
any question in ש"ס and he will have an answer.
They asked him from the גמרא נדרים דף ח a+b
"If someone dreamt that he was put in חרם and
they were also מתיר his חרם , he must be מתיר
his חרם again after awakening.
Because we must assume part of the dream might be true and part not to be true, We assume the true part of the dream was being put in חרם and the untrue part of the dream was they were מתיר him".
They asked from the Rosh Yeshiva,
"why don't we assume the opposite, that the untrue
part was being put in חרם and the true part
was them being מתיר the חרם, and he wouldn't
need another היתר after awakening?
The R.Yeshiva answered that according to
the ראשונים who hold חרם is דאורייתא and even if it is
only דברי קבלה, when there is a ספק we need to go
לחומרא and assume he was put in חרם and didn't
have a התרה
The Steipler said, " from the answer he gave, you
can tell the Rosh Yeshivah was drunk."
The question doesn't even begin
( no need for the Rosh Yeshiva's answer}
The only possibility for part of the dream to
be true is if he was put into חרם.
If he wasn't put in חרם (untrue part) then
the התרה will also be untrue since there
was no חרם there is no התרה.