There's no bug.
The dot '.' is used to quiet output, this mean that nothing will be print out.
If for example you do //timer 1 1 noop, it will echo in you window :
* Timer 1 activated
* Timer 1 halted
If now you do //.timer 1 1 noop, you won't see this message.
In the situation above, mirc parse $network" as an identifier and simply, either tell you that it does not exists if you have enabled the option "identifier warning" (as an error message), either it return the $null value.In both case, mirc will stop because it's either an error and mirc stop, either the double $ make mirc stop because he his seeing $$?="... (this return $null too).This is why RusselB never see any error, not because he doesn't use his feature often.
