When a Moodle site is misconfigured, it is possible for the function `guest_user` to return `false` instead of a user object, although most of the code I've seen assumes that `guest_user` will always return a valid user.

This is likely happening when `$CFG->siteguest` was overridden but invalid, as deleting the guest user should not be possible.

I would suggest to either change `guest_user` to always return a valid user object, and throw an exception when it cannot find one, or check that `siteguest` is valid when it is set in the config file.

(This was identified from a bug reported in Level up!)