jboner/akka — comment on commit 4b8f11ea92ab3fb4809e2ca84b11ac93771477bb
File: akka-actor/src/main/scala/akka/actor/ActorSystem.scala line 55
By havocp:

chaining withFallback() like this unfortunately does not do the right thing right now; I think I have to fix the API or implementation somehow, since this is the obvious way to use it. I wrote some docs explaining the problem yesterday, see withFallback doc comment in ConfigMergeable. Basically if you have in three configs: foo={}, foo=null, foo={} and you do first.withFallback(second).withFallback(third) then the result of first.withFallback(second) is foo={}, and then the foo in third gets merged in to the foo from first because when merging third, the foo=null has already gone away. but really because of the foo=null in second, anything in third should have been ignored. It is a minor corner case but you'd use foo=null if you wanted to "clear" an object found in a fallback. Anyway I guess you don't need to change this, I just need to make it work instead of documenting a crazy workaround. I'll fix it.