One of the funny symbols of mathematics textbooks.
The lightning bolt symbol
may be written inline with a proof by contradiction
at the point where the assumption of the inverse has
finally led to a reductio ad absurdum (eg, they show
that 1=2, or another such obviously false statement).
Not an end of proof symbol technically, since there is
usually one or two more sentences to the effect that since
there is a contradiction, their theorem holds.
In a proof with lots of cases, you could wander into
a veritable lightning storm of a page.