Example code (full program):

typedef int v4si __attribute__ ((vector_size (16)));
int main() {
	v4si x;
	x[0] = 33;
	return 0;
}

Compiling with gcc produces a working program and no console output, but g++ complains:
$ gcc -Wall -Wextra vecbug.c 
$ g++ -Wall -Wextra vecbug.c 
vecbug.c: In function ‘int main()’:
vecbug.c:4:5: error: invalid types ‘v4si {aka __vector(4) int}[int]’ for array subscript

Verbose output:
$ g++ -v -Wall -Wextra vecbug.c

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6.1/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.1-9ubuntu3' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3) 
COLLECT_GCC_OPTIONS='-v' '-Wall' '-Wextra' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.6.1/cc1plus -quiet -v -imultilib . -imultiarch x86_64-linux-gnu -D_GNU_SOURCE vecbug.c -quiet -dumpbase vecbug.c -mtune=generic -march=x86-64 -auxbase vecbug -Wall -Wextra -version -fstack-protector -o /tmp/ccNwRhl7.s
GNU C++ (Ubuntu/Linaro 4.6.1-9ubuntu3) version 4.6.1 (x86_64-linux-gnu)
	compiled by GNU C version 4.6.1, GMP version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.6.1/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/4.6
 /usr/include/c++/4.6/x86_64-linux-gnu/.
 /usr/include/c++/4.6/backward
 /usr/lib/gcc/x86_64-linux-gnu/4.6.1/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.6.1/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C++ (Ubuntu/Linaro 4.6.1-9ubuntu3) version 4.6.1 (x86_64-linux-gnu)
	compiled by GNU C version 4.6.1, GMP version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 63a745aba0054d1605090177d8e4bbb4
vecbug.c: In function ‘int main()’:
vecbug.c:4:5: error: invalid types ‘v4si {aka __vector(4) int}[int]’ for array subscript