There is an unreleased self-host regression where requiring a namespace while in that namespace triggers circular dependency detection.
As a concrete example, let's say you are in a REPL, and you require a namespace, go into that namespace (using in-ns), exercise it a little, and then change the code to fix something and then reload it. This now fails on master for self-hosted code.
A repro following this example is the following:
This causes the following on master, but not with the 1.9.908 release:
(Strictly speaking, the above example is not minimal in that :reload is not required in order to reproduce it: It will happen if you simply attempt to require the namespace while "in" it.)