You will be able to spot and avoid the common tagging errors that split or overwrite your data.
Before you send your next tagged link, open a private browser window, paste the link in, and check that your visit shows up in GA4 with the source and medium you set. The test takes about two minutes per link. Each mistake below can make a report wrong without anyone noticing. The test catches some of them before the link reaches anyone, and a few habits you set up once prevent the rest.
When a visitor clicks a tagged link, GA4 treats the tags as a statement of where the visit came from. For a link in an email, that statement is correct. For a link inside your own website, it's false, because the visitor was already on the site. If a visitor clicks an internal tagged link, GA4 keeps the session's original source and medium, but it attaches the new tags to the events recorded after the click. GA4 uses those events for event-based attribution, so a purchase after that can be credited to the internal tag instead of the real original source.
Farah owns a business that sells work trousers. She has a newsletter and a homepage banner for new work trousers. Her assistant wanted to find out how many people clicked the homepage banner, so the assistant tagged the banner link with utm_source=homepage and utm_medium=banner.
A newsletter subscriber then clicked the newsletter link, landed on the homepage, and was recorded as newsletter / email. The subscriber clicked the banner, and GA4 read the new tags. The session stayed newsletter / email, but the events after the click were recorded with homepage / banner. A few minutes later the subscriber bought trousers. In event-based attribution, the purchase was credited to the banner instead of the newsletter. Nothing had changed in the newsletter, but a month later (an example timeframe) Farah saw in GA4's conversion reports that newsletter sales had almost vanished and a new source, "homepage", was suddenly one of her best performers.
Only put tags on links that point to your site from somewhere else. Never tag links between pages of the same site. Links in your own online shop's emails, such as order confirmations, are fine to tag if they point back to the site, because they come from outside. To measure clicks on your own banners and buttons, use GA4 events, which module 3 covers. Events record the click without changing where GA4 says the visitor came from.
Tag values are case-sensitive, so "Email" and "email" count as different values. Lesson 2.2, "A naming convention your whole team can follow", explains why. Inconsistent values usually build up when several people tag links over several months. One source might appear as "insta", "instagram" and "ig". A freelancer who never saw your rules might change a campaign name from "2026-11-work-trousers" to "nov-trousers".
Each spelling becomes its own row in the report, so one campaign's results get split across rows and any single row looks weak. A campaign with 400 visits might show as five rows of 80 (400 / 5 = 80, an example figure). No error message appears, so you only see the split if you go looking for it.
To prevent this, use the shared rules page from lesson 2.2 and the tagging sheet from lesson 2.4. Then check once a month. Open the traffic acquisition report in GA4, sort sources and campaigns alphabetically, and scan for near-duplicates. Sorting puts misspellings next to each other, so they're easy to spot.
Tagged links are long and ugly. Pasted into a WhatsApp message or an Instagram story they look like spam, and people are less likely to tap them. Some people trim the tags off before sharing, which defeats the purpose of tagging.
Use a link shortener instead. It gives you a short address that redirects to the full tagged link, so the tags still reach your site. Many website builders and email tools have a shortener built in, and free stand-alone shorteners also exist. QR codes work the same way: generate the code from the full tagged link, or from a short link that redirects to it.
Take two precautions with shorteners. First, use the test in the next section to check that the shortener passes the full link through. Second, if you use a free shortener and later lose access to the account, every printed QR code pointing at it can break. For printed material, use a service you control or a short page on your own domain.
A tagged link with a typo still works for the visitor, so nobody complains. GA4 files the visit under the typo or under direct, and you may find out weeks later or never. The GA4 realtime report shows activity from the last few minutes, so you can use it to check a link before it goes out:
Open a private or incognito browser window. Paste the tagged link. Check that the page loads and that the tags are visible in the address bar. Open GA4 and go to the realtime report. Within a minute or so, confirm that a user appears with the source and medium you set. If it shows direct instead, something stripped the tags, usually a redirect or a shortener. Fix it before you send the link.
Once you set up an internal traffic filter in module 3, GA4 excludes your own visits, so your test visits may stop showing in realtime. Lesson 3.2 explains how to test with the filter in place.
Farah now tests the newsletter link, the bio link and every new QR code before they go out, and she hasn't lost a campaign since. Running the test takes less time than reading about it did. The first time you see your own visit appear with the right labels, the tagging system starts to feel concrete. Take the next tagged link you plan to send and run it through the test now.
Click one of your tagged links. Find the visit in GA4's realtime report and write down the source and medium it shows.
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