99% Invisible doesn't work in some Mazda cars. The person who wrote the code could only say that the % sign had some meaning in the URL and that caused the "heartbeat" to tell the car that something wasn't working and therefore to restart.
But no understanding of why this happens to % I and not % with other letters. Even % i -- the lowercase -- is fine.
Wow. I'm guessing the parsing of the .xml feed file is badly broken? If the string was getting passed somehow as the first argument to something like sprintf(), I could see a crash happening, but the first argument should be a fixed format string, not incoming data.
Weirdly, when they simply changed the title from "99% Invisible" to "99% invisible" that solved it.
"% i" might(???) be interpreted as "print a decimal number without including the sign," where "% I" is (I think?) not a valid format string. This is pretty speculative though. And it still doesn't make a lot of sense to me that part of the file name would be interpreted as a format string.