Some websites abuse the History API to inject history entries that break the Back button.
For example, on Safari on iOS 15.1:
1. Visit google.com and search for "PVR Silver Arc"
2. Tap on "Website" in the search result box for "PVR Silver Arc".
3. Tap on the back button to try to return to the search results page.
The site has inserted an entry into the back/forward list so no matter how many times you tap on the back button, you stay on this site.
Blink and Gecko have shipped an intervention to prevent this kind of abuse. This marks entries added to the Back/Forward list without user action so that they're skipped when tapping on the Back button.
Blink bug: https://bugs.chromium.org/p/chromium/issues/detail?id=907167
Gecko bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1515073