... what we can do to prevent these types of "side-effects" when people are touching the config system.
The use of defconfig files implies that we never change default values of configuration. If someone does change the default value of a configuration setting, then it effects ALL configurations that depend on that default. We need to catch this is the review step. During review, if we notice that there is a change to the default value of the configuration setting, then we must insist that all defconfig files be modified so that the previous configuration is not changed due to the side-effect.
CI cannot catch this. As noted, the error does not occur until run time. A proper regression test would catch this but we do not do any automated regression testing. That is a major hole in the testing.
