basically it means that the option is not 100% reliable -_-
As mentioned, it has happened to me that stuffplug said that someone opened a conversation, then i asked my contact if he did and he said no... this is because, as explained there, sometimes MSN makes a connection between 2 contacts without someone opening a conversation... what Stuffplug does is detect such connection, so it will be wrong sometimes when saying that ur contact opened a convo with u... but stuffplug is almost alays right so dont worry at all