Activity/step blinks red in the diagram, and the workflow goes to the Paused state (default behavior.)
The workflow Failed flag is set to Yes (true) which can be seen in an Explorer folder or view containing workflows or in the Home page Instance monitoring dashboard.
An uncaught exception during the processing of the activity/step.
To diagnose the exception, right-click on the activity/step and select Display logs. The fix will depend on the particular exception.
In the future, avoid taking the action that caused the exception.
Specific exceptions will be described in subsequent discussions of this sub-forum.