6.4.3.5 For any text file t, the effect of attributing to a component of either t.L or t.R a member of the characters prohibited from text files is implementation-dependent.
In other words the effect of writing, any of the characters that implementations are allowed to prohibit from text files, to a text file is implementation-dependent.
Irie Pascal prohibits one character from text files, the character is chr(26) the end-of-file character. Writing the end-of-file character to a text file causes the file to be terminated (i.e. the EOF character and any characters written after the EOF character will not appear when the file is read).