Problems

The Flow Designer will check the workflow for possible errors at regular intervals (or if the F5 key is pressed). Errors are displayed in a list which as of default is located below the workflow, and workflow steps with errors are rendered with a red background.

Workflows with errors can not be published. Workflows with warnings can be published but is not recommended.

Most type of errors/warnings can be double clicked and it will highligt the failing step. Some errors/warnings is related to more than one step and in that case a search will be performed when double clicking that row, the search result will be shown in the Find Panel

Example with both errors and warnings