It may be the error of a scripter who put something like that in for testing purposes and forgot to remove it (as I have done myself >.<). To see if this is what it is, go into your script editor and press Ctrl + Shift + F, type in msgbox and look at the results. By default, there are only two instances where this is used, once in DataManager and once in Game_Interpreter. If there are any other instances of this (of course, they would only be in custom scripts), post the script here and I'll check it out.
Does this occur every time you call any common event?