Given the wide variety of inputs to a STEM model, and the complex and diverse relationships between those inputs, there is enormous scope for inconsistencies to arise as you develop a model. Trivial errors are intercepted as soon as you enter them. For example, if you mis-type an input or enter a string when a number is required, STEM will demand an immediate correction.
However, there are several other categories of checking which are deferred, so that you can continue to work on your model, even though there may be outstanding problems to resolve. STEM automatically checks models before running them, but you may prefer to check your model periodically, to avoid having to confront a significant backlog of errors when you do come to run the model: