In the winning entry there is no cast or "conversion" per se. It has one C file that calls a function and another C file that implements the function, with a mismatch between the types of the call and the implementation. Neither file by itself is performing any conversion or doing something wrong that can be caught by static analysis; the bug is caused by a mismatch between the code in two object files. This would only be caught by a tool that would examine the two files together, but it would not be caught by the compilation of either part.
We've actually seen a number of past entries that used this same basic trick to mismatch a call and an implementation. A previous winning entry managed to redefine the time() function as time_t time(void) instead of time_t time(time_t *ptr), avoiding a compiler warning by using the extern keyword. That's a neat trick because barely anyone uses the argument to time(), and after writing t=time(NULL) hundreds of times, it's easy to completely miss a call like t=time(). This caused a call to time() with the wrong number of arguments, so that another variable on the stack was used to hilarious effect.