When flying on VATSIM it often happens that you create a flight plan on SimBrief, select ‘Prefile on Network → Vatsim’, go to submit the flight plan on VATSIM only then to be informed ‘Callsign in Use’. You then have to edit the flight plan on SimBrief, change the ATC callsign, regenerate the flight plan then attempt to refile.
It would be good if there were an option for SimBrief to warn you if the callsign you have typed in is already logged in on VATSIM (or potentially other networks as well, in a user-configurable manner).