Check model is the designer feature that helps you find mistakes in the model. Usually they are easy enough to understand, but there are times when things to wrong and it's not easy. Some odd thing "gets stuck" or you have merged the model in your repository and you get XML errors in the saved model, like below;

This is really difficult to find if looking at all the model files.

But, the model debugger to the rescue.

Open the model debugger by right clicking on a diagram background and select Extras and Open Model Debugger...

You can then click to open the object editor to investigate, usually find where the problem lies, or at least delete the offending model object using EAL.