No, actually I would have been happy to use echo's around where I use scid but I didn't know how to duplicate the issue. But this has happened again now and I did put echos and recieved nothing in the window. Only the same error message.

Which proves that you've got another /scid somewhere in your code that you don't know about. You may be 'pushing mIRC more than most people' or not, but if this error wasn't directly from the /scid command then that wouldn't matter, there would be a lot of people seeing this if it were a mIRC bug unrelated to that command. Remember it needn't even be plaintext "scid" in your code, it could be a dynamic command call such as "%cmd blah blah" where the variable (or identifier) supplies the command. Or it could be a timer (I know you said you've checked but the timer could be intermittently created), or the command could be being provided to mIRC via DDE, COM, or a DLL. It could even be the result of an exploit in your code where someone is trying to execute that command remotely (although admittedy it's unlikely that they'd be using /scid).

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