Like Tom says, this happens when a different PLC has been opened and a Send & Receive has been attempted. This isn't necessarily always a bad thing.
PLCs can be saved and saved as and saved as, leaving you with multiple copies of the same PLC. You can then open any one of them and make the desired changes and send and receive the changes back to the model. Sure you'll get this warning, but it's just a warning. What this means is that we can work on multiple options of the same PLC and send back whatever option we want. It takes some due diligence, but it is a brilliant way of managing different options, and can be used on any type of project. I teach this all the time and it has proven very popular.
I'd say in your case, mate, that it may be OE (operator error
), and that you may want to get all of your PLCs in a row. It doesn't mean that you can't go back and send & receive with another PLC as long as you don't sign out.
Cheers,
Link.