Compiler warnings can come down to things that are purely style, like unused variables. It's more than likely that an unused variable could sneak itself into a

Compiler warnings can come down to things that are purely style, like unused variables. It's more than likely that an unused variable could sneak itself into a program, not be found for a few years, then suddenly trigger a warning when a compiler added the check (of course, compilers have been checking for this for a long time, but this is just an example). In the case of Gentoo, this would manifest itself as packages compiling cleanly with one compiler version, then suddenly lots of packages failing to compile because the build processes were stopped due to the, now reported, unused variable. If these were just warnings, then the packages would still compile, someone would notice (or even the original dev), and the problem can be fixed. Note that before the compiler upgrade, there was no bug - the program worked fine. Except for a case such as libressl (preview release so they can get comments), having -Werror hardcoded in the build process clearly makes no sense.