Technical Pickles

I've often seen people submit ebuilds with src_unpack() like:

src_unpack() {
    unpack ${A}
    cd ${S}
}

This is not needed because there is a default src_unpack() that effectively does this for you. Take a look at /usr/lib/portage/bin/ebuild.sh and see for yourself.

Update: Diego pointed out one case where you would want to override src_compile with the default: you inherit an eclass that provides a src_compile that you don't want to be used. This is probably a pretty rare case though.