Memory leaks within a C extension are even worse. You'll see a lot of tools and articles about finding leaks in Ruby. However, you don't have the same access to internals in C.
A naive usage of rb_funcall can cause memory leaks: it's much better to use rb_protect instead. So, if you are a C extension writer, please read on for the sake of developers who will use your gem.
rb_funcall can be a great tool when you need to interact between Ruby and the C parts of your library but only need to write a little C.
However, when you run rb_funcall, you are no longer in C where everything is straightforward. You can be left in muddy waters if the called function:
Number 1 is the easiest one to catch. You'll likely end up with a segfault, and if your test suite is complete enough, you should catch that before publishing.
Ruby's raising mechanism jumps between parts of the code from one scope to the first parent that catches an error. This is implemented in the MRI using longjmp and setjmp.