Summary

The article lists 21 common mistakes in using Google Tag Manager (GTM), including not publishing the container, viewing the wrong GTM container, misusing data layer variables, not using debugging tools, not removing hardcoded tracking codes, not auditing, improper use of dataLayer, not monitoring page performance, typos, lack of cooperation with developers, not using Regex, publishing all changes at once, improper form tracking, lack of naming conventions, not adding event settings variable to tags, not labelling container versions, misplacing event parameters and user properties, lack of planning, and overlooking container backup importance. The author advises learning from these errors for improved GTM usage.