Handling of structural errors
When an expression fails for a structural reason, e.g., due to a removed custom field that is still referenced in your expression, you will see these errors in the global error list. It will show up in the individual error list of the respective condition or validator, as well. You have the possibility to resolve these errors such that they do not appear in the global error list anymore.
This is only true for structural errors, i.e., if you have an expression that simply evaluates to false this will not show up as an error in any of the error lists.
You should fix these errors as soon as possible as the affected transitions will not work anymore if they fail for a structural reason.