See this rule, for example:
install_target: first FORCE
@$(CHK_DIR_EXISTS) $(INSTALL_ROOT)/usr/bin/ || $(MKDIR) $(INSTALL_ROOT)/usr/bin/
-$(INSTALL_PROGRAM) "$(QMAKE_TARGET)" "$(INSTALL_ROOT)/usr/bin/$(QMAKE_TARGET)"

Due to the leading dash on the command, "make" ignores any errors and returns 0, so scripts or other tools caling "make install" cannot know that it failed, even though it seems rather obvious that they should.