I understand you are talking about the on-the-fly calculations, also called Partial Results in the Settings. It is simply a real-time calculation of the currently inserted expression. If you type a '+' at the end, for instance, the "Compile error" message will be shown because there is an error in the expression (it ends with an operator). Although this message could be improved, there is nothing to worry about. Also, you can disable the yellow box in (English-version) Settings > BEhavior > Partial Resulls.