Recently Google Tag Manager has released the new Preview mode version, where you can test your tags, triggers and variables before publish them on your website.
In this new version, you’ll see that a new URL it creates on your website.
Here you can see an example when I launch the Preview mode:

Once the process is done, you can see the new URL:

If you do not fix this in your Google Analytics Admin, you’ll find the page into the reports.
This’ll happen both on the Universal Analytics version:

And in the new Google Analytics 4:

What’s the problem?
In this way, GA will process this information by showing you data, which will not be very accurate.
The total number of pageviews will be altered by the pageviews that are coming from an URL which is only useful for test your implementation before it’s published.
Not very qualitative, right?
Let’s imagine if you are not the only ne who are using the Preview mode. Your Google Analytics will continue to collect lot of useless information.
How exclude “gtm_debug=x” in Universal Analytics
Let’s go to Admin section and click on the View Settings. Select the View where you want to exclude the value “gtm_debug=x“:

Now it’s time to fille the value “gtm_debug” into the Exclude URL Query Parameter entry (if you had more than one values, you can simply add a comma):

The value “gtm_debug”is a query parameter (if you want to learn more about it, let’s read my post or discover how to find other query parameters in Google Analytics) and it needs to be excluded by typing it into the box above to avoid being collected.
Tip: the best way to do this, is to implement the solution into the Test View and then, if all it works fine, into the Master View.
How exclude “gtm_debug=x” in Google Analytics 4
The new version of GA, called Google Analytics 4 doesn’t have the views as in Universal Analytics (if you want to know how to implement GA4, read here).
We have to find another way to exclude the query parameter “gtm_debug”. You have also to consider that who works on the preview mode with GTM is an employee, yourself or an agency.
We can define these three sets as an “internal traffic” and therefore use the filter provided under the GA4 Property: Data Settings > Data Filters > Internal Traffic:

You can simply follow the steps described in this post to exclude internal traffic and thus also the page with the value “gtm_debug=x”.
Exclude “gtm_debug=x” in GTM
Below there is another option you can choose. The solution could be to disable the “Include debug signal in the URL” entry as in the image below:

This is the weakest option as you can read in the red box.
Conclusions
It is important to pay attention to these settings. Even if the values collected seem minimal and do not affect the data collected by GA, you should always avoid having Analytics process data that is not useful for analysis.
Checking or auditing your Google Analytics account at least 3/4 times every year is a good procedure in order to be sure that GA is collecting “good” data.
Good analysis!
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