With this testcase:

int foo (int (*cb)(void)) { return (*cb)(); }
static int callback(void) { return 0; }

int main (void)
{
 int callback (void);
 return foo (callback);
}

On x86_64 with -fPIC, the address of callback as passed to foo is incorrect.

If callback isn't static, it works (through the GOT).  If the declaration of
callback is removed it works (computes the address as %rip minus something).  As
written, it does a failed combination of the two.  The result is that instead of
passing callback's address, it loads the first 8 bytes at callback's address and
passes those.  (It also erroneously claims callback is unused, but I'm guessing
that's a side-effect.)

The code is questionable, but my reading of the standard says it's legal and
that callback is still static after the (implicitly extern) declaration.

% gcc-3.4.4 -O2 -fPIC -Wall -save-temps -o bug bug.c 
bug.c:2: warning: 'callback' defined but not used

% gcc-3.4.4 -v
Reading specs from /usr/local/lib/gcc/x86_64-pc-linux-gnu/3.4.4/specs
Configured with: /tmp/src/gcc-3.4.4/configure --enable-languages=c,c++
--enable-version-specific-runtime-libs --disable-shared --enable-threads
--with-gnu-as --with-gnu-ld --with-pic --host=x86_64-pc-linux-gnu
Thread model: posix
gcc version 3.4.4

[I bootstrapped this 3.4.4; it also fails with stock 3.4.3 from RedHat.]