Thanks to your assessment, I think I know what caused this!
I was trying to help someone with a macro and part of testing it required exactly the setup you described.
The macro I was testing triggered two test macros. Each of these contained only an execute a macro action, and each triggered the other. One action would be active at a time so as not to incur a feedback loop, but there was another issue lurking, as we've now discovered!
I'm not sure how it makes me clever, but it certainly makes me accident prone!
If it helps in any way, I think the macros in question are most likely to be in either my "TEST GROUP" or "KM Forum Requests" group. Oddly, I can't imagine why I wouldn't have deleted those test macros after posting the macro being tested to the forum but hey-ho...