I partly agree. -V was made for hassle-less running of stuff you don't have time to get up to date right now. And I suspect the sprintf stuff might be major there.
On the other hand we've OK:ed bugfixes without requiring backwards compatibility before, where it was a clear bugfix. But when it was a bug fix in Pike, not a detecting bugs in the program pike is trying to run. So I'm not sure I think it's a blocker, but I'm concerned.
The question to grubba is: Can backwards compatibility be fixed without spending weeks on it and without losing something (like performance) in the process? A warning instead of an error would be might nice for code like this, but the primary concern is running the code, not running it better than it used to run on the Pike the compatibility layer is emulating.