It's throwing this error on my test site:
Fatal error: Out of memory (allocated 267386880) (tried to allocate 65484 bytes) in /path/to/wp-content/plugins/wp-simple-firewall/src/icwp-firewall-processor.php on line 326
But it didn't throw a 500 Server Error this time.
Which is strange because I'm on a dedicated server with lots of memory to spare. This is also a test site with only one other plugin activated and a commercial theme activated.
It works fine if I check this:
Ignore Administrators
Ignore users logged in as Administrator
I'll keep poking at it, to see what else comes up.