This reminds me, somewhat tangentially, of a bug I ran into once with Tcl (yes, Tcl) where a function returned an integer in the form "08" and Tcl had a convent

This reminds me, somewhat tangentially, of a bug I ran into once with Tcl (yes, Tcl) where a function returned an integer in the form "08" and Tcl had a convention that any integer with a 0 prepended would be treated as octal in any arithmetic expressions. Naturally, 8 is not a valid octal digit, so this broke everything when it happened. Ultimately it was my fault for not being better acquainted with the language, but that one still caught me by surprise.