I experienced issues with this flag being used by an open source library in autotools (specifically the AM_INIT_AUTOMAKE option in the configure.ac file). The actual C code also used -Werror, and compiled perfectly, but "./configure" wouldn't complete because of minor "this is a GNU extension" and similar errors during automake.

Not being an autotools guru this was incredibly frustrating. I had no idea why the damn thing wasn't working until I noticed "warnings are treated as errors".

Please don't use -Werror in any context if your project is open source.