Has Ocaml any unsafe features?  With "unsafe", I mean constructs
which, when (ab)used in certain ways, make further reasoning about the
behavior of the program impossible (and neither the compiler nor the
run-time system provides a diagnosis).  Examples in other languages
are out-of-bounds array access, null pointer dereference, integer
arithmetic overflow, premature deallocation of heap objects, wrong
type declarations -- you get the idea.

The C language interface falls into this category.  Are there any
other problematic areas?