View > Show Errors Window
The Errors window displays warnings and errors in the project, the error or warning will remain in the window until it is fixed.
For example, in this particular project the maximum image size of the project has been set to 4096 x 4096 pixels (see Preferences).
The input image has a size of 500 x 375 and the Transform node applies a scale of 10 x 10, generating a new image of 5000 x 3750, larger than the maximum size image.
In this case the Transform node reports an error to the Errors window and the title of the node is displayed in red.
To fix the error set the scale of the Transform node to a lower value. As soon as the node reporting the error evaluates successfully the image the error is removed.