The term "writing on the wall" is now proverbial. People say things like, "He's seen the writing on the wall," or "Yo—time to read the writing on the wall, kid" or "Aghh! There's a magical hand writing on that wall!!"
That last one isn't something people typically say, but it is something Belshazzar might've said when he actually saw "the writing on the wall" the first time (though he would've said it in Babylonian, presumably the "Aghh!" would've been the same in any language).
The "Writing on the Wall" story is different from the Nebuchadnezzar stories. Nebuchadnezzar has dreams that tell him about the future, but he gets an escape hatch. Since he never does anything quite as bad as drinking from sacred vessels dedicated to God, the Nebster is able to get off with a mere seven years of insane wandering in the wilderness as an animal, without the power to speak. But when he regains his sanity—it's all good.
But for Belshazzar, this psychic phenomenon of magical writing is meant to indicate one thing: your fate is sealed. He has no escape hatch—it's over, baby. That's what the "writing on the wall" term has come to mean when we use it in the present day world too. You need to recognize what the inevitabilities are if you're going to face up to the truth.
Anyway, this story also proves that the original graffiti artist was God, who scratched out some threatening Hebrew letters on Belshazzar's wall. It's not exactly Banksy, but pretty good stuff, nonetheless.