If you see a referee hold up a "X" symbol above their head with their arms, it means a wrestler has been injured in the ring and the match needs to come to a stop before its pre-determined conclusion.

HISTORIANS of mathematics attribute the first use of the cross x as a symbol for multiplication to William Oughtred (“Clavis Mathematicae”, London, 1631). See W. W. R. Ball's “Short Account ...

An X can appear across a square symbol, on top of an iron picture or underneath an iron, and over a circle - so there's a likelihood you have seen an X marked on a few of your garments.