I've decided to write some evil tests for sprintf() function. I'm not sure if they're valid because spec says nearly nothing about sprintf() aside of format used by it (and "is mostly identical to the C library sprintf function", even if it has vectors in grammar which only Perl 5 implementation has), so I'm making pull request.
More evil sprintf() tests
+1. The spec needs clarity on exactly what is and is not a valid sprintf format string as well as how errors will be thrown.