I remember that there is one way to create an invisible charater by typing 4 digits while holding Alt + X when renaming a file. What is the 4 numbers?

Alt + X + ????

its not Alt + X + ????
its Alt + 255
as in Press and hold Alt, Type 255(must be typed on Numpad), Release Alt.

i can't make it. this is the outcome. i did typed in Numpad.

ÿ

i remember that i was taught to type Alt + X + ????. by a video...

Oh i found it myself. It's

Alt + 0160

Adding X or not doesn't make any difference.
Thank you guy anyway.

alt + 0160 works regardless (it's unicode)
alt + 255 will generate either: the exact exact same character, or y with a umlaut: ÿ.
If typing alt-255 makes a non-breaking space for you, try alt + 0255 and you'll see...
It all depends on the code page being used (like Windows-1252)

I became wary of using ALT + 255 for use in filenames, because it is exploitable, being the ASCII code that the keyboard sends when you press the DELETE key.

That's not the case at all. Keyboards only send make & break codes. It's not even close to being the same.

Oh so that is how those AOL hackers did back then.

other then that when I try 255 it give me _

Great now I have a bunch of heads on my desktop....