Before going ahead with the debug log, the one thing you have to do is set the correct log filters, the steps for which are as follows:

1. Navigate to Setup | Monitor | Logs | Debug Logs.
2. Click on New available under the Monitored Users related list. Using a magnifying glass, add the users whose debug logs you would like to monitor and retain. In the current case, select your name.
3. Click on Save.
4. Navigate to the Monitored Users related list and click on the Filters link to change the debug logs filter. There are mainly three kinds of setting available:
   • Log category: The type of information logged.
   • Log level: For the Workflow and Visualforce page, set it as FINER. If you set the log level for the running user to be FINER, it will show your assignment actions and values. 
   • Events: This is the combination of Log category and Log level.
5. Click on Save.

For example, if you try to run the Create Leads Flow that we created in Chapter 3, Manipulating Records in Visual Workflow, from canvas Flow by clicking on the Run button, then you will receive a runtime error or fault error. If you go to the debug log and open it, you can easily identify the reason behind it: the campaign ID is null (inside Flow, the Record Create element, Add_to_campaign). This is because we are passing the campaign ID through the custom button and not through the Flow, as shown in the following screenshot:

Through the debug log, you can easily track a bug or error in your Flow.

If Flow creators want to receive an e-mail for each fault/error, whether it's the same or not, enable critical updates, Flow Creators Receive Email for Each Unhandled Fault.