call_out() saves and restores the current user. It is now possible to use say() or write() which rely on a current user to be something useful.
The execution of the call_out()s implies a simple (not exhaustive) measure against rabbits: the evaluation costs of those call_outs() executing at the same time are summed up on a per-user base. If the summed-up costs exceed the given maximum, a 'too long evaluation' error will occur and any remaining call_outs() of this user scheduled for the same time are discarded.
call_out("RefreshMe", 10);
This will call the function RefreshMe() in 10 seconds without
any arguments. The function RefreshMe() can then call out
itself again which will result in a loop (not in a recursion)
which can be used to check or set up things in the object in
intervals. Be aware that callouts are stored in a linear
list, and so are somewhat expensive for the driver.
And YES: self-replicating call_out()s, where each call_out()
creates two or more other call_out()s in a loop (so called
'rabbits') slow the mud down very fast, and are even able
crash it. No need to try it yourself.