1. The model uses click-through data as an input. Your frontend engineer moves the UI element being clicked upon to a different portion of the page for a certain category of results. This changes the baseline click-through rate. The model assumed this feature had a constant baseline across all results, so the new feature value now needs to be scaled to account for the different user behavior. Nobody thinks to do this.

2. The frontend engineer removes a seemingly-wasted HTTP fetch to reduce latency. This fetch was actually being used to calibrate latency across different datacenters, and was a crucial input to a data pipeline to a system of servers (feeding the ML model) that the frontend team didn't control and wasn't aware of.

3. The frontend engineer accidentally triggers a browser bug in IE7 (gimme a break, it was 9 years ago) that prevents clicks from registering when RTL text is mixed with LTR. Click-through rates decline precipitously in Arabic-speaking nations. This is interpreted by an ML model as all results being poorly performing in Arabic countries, so it promptly starts cycling through results, killing ones that had shown up before with no clicks.

4. A fiber cable is cut across the Pacific. This results in high latency for all Chinese users, which makes them abandon their sessions. A ML model interprets this as Chinese people being less interested in the news headlines of that day.

5. A ML model for detecting abusive traffic uses spikes in the volume of searches for any one single query over short periods of time as a signal. Michael Jackson dies. The model flags everyone searching for him as a bot.

6. A ML model for search suggestions uses follow-up queries as a signal. The NYTimes crossword puzzle comes out. Everybody goes down the list of clues and Googles them. Suddenly, [houston baseball player] suggests [bird sound] as related.