I don't remember much specifics, but IIRC, most of the implementation related ones were due to an anti-pattern from the older a/b testing framework. Basically, the client would try and determine if the user was eligible to be in the A/B test (instead of relying on the framework), then in an API handler, get the user's assignment. This would mean the UI would think the user wasn't in the A/B test at all, while the API would see the user as in the A/B test. In this case, the user would be experiencing the 'control' while the framework thought they were experiencing something else.

That was a big one for awhile, and it would skew results.

Hmmm, another common one was doing geographic experiments when part of the experiment couldn't be geofenced for technological reasons. Or forgetting that a user could leave a geofence and removing access the feature after they'd already been given access to it.

Almost all cases boiled down to showing the user one thing while thinking we were showing them something else.