That's cause backslashes are not escaped in string literals in VC. But are escaped in character literals!
In the meantime, try this (ugly) fix!
Textbox("You found a " + chr('\f') + "1Potion" + chr('\f') + "0 in the chest!")
chr() takes an int, ints can be expressed as character literals (which allow escape codes already). It's ugly magic. The same thing needs to be done to put \n newlines into text for prints.
Someone should really fix strings, one day soon. ;_;
The problem is, a lot of old code used backslashes without escapes, since there was no such thing as escape sequences back then. Especially in line-wrappers before we added WrapText(), and in choice-menu systems where they used \ as a special character.
I think we're at the point though, where we can add escape sequences to the language (possibly add a config flag for backwards compatibility).