I study a lot of file formats, and sometimes I come across a quirk in a format, or its documentation, that isn’t worth making a fuss about, but which might still be interesting to two or three people in the world. I suppose this one doesn’t even meet that criteria, though it is in a very well-known format.
CompuServe’s GIF image format specification, version 89a, says:
25. Plain Text Extension
a. Description. … The textual data will be encoded with the 7-bit printable ASCII characters.
It does not define “7-bit printable ASCII”, but that almost always means ASCII characters from 0x20 to 0x7e.
Then it says:
e. Recommendations. … If characters less than 0x20 or greater than 0xf7 are encountered, it is recommended that the decoder display a Space character (0x20).
The appearance of “0xf7” here seems inexplicable. I can only guess that it is a clerical error that was supposed to be 0x7f. Which would make it an error within an error, because (in most or all forms of ASCII) 0x7f is a nonprintable control character. So it really should be 0x7e.
Even if this error had some consequence, which it doesn’t, it still wouldn’t matter, because the Plain Text extension feature is obsolete.