malloc() and printf(), for example, have mutexes internal to their implementation. Suppose the parent process has two threads. One is about to do a fork, and the other is in the middle of a malloc(). The fork occurs. The parent process continues on as normal. In the child process, there is only one thread -- a clone of the forking thread, but the memory state of the child process is a full clone of the parent (except for the return value of fork(), of course).

The single thread in the child calls malloc(). But the malloc mutex is already held, because in the parent process, a thread was executing there. Unfortunately, since that thread does not exist in the child, it will never be able to release the mutex. The thread in the child is deadlocked.