the "debug" USE flag has a description: Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see http://www.gentoo.org/proj/en/qa/backtraces.xml referenced link doesn't work, the link should be updated to: https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces system wide
Fixed, thanks! + 31 May 2015; Jason Zaman <perfinion@gentoo.org> use.desc: + update debug url to wiki, bug 550874