[wat10000]

There’s an obvious example of this with twin-engine airplanes. Having two engines obviously makes you a lot safer since you still have power if one fails. But dealing with an engine failure takes some skill, and your probability of experiencing a failure doubles. Airlines train their pilots to handle it, but if you’re a more casual pilot and you’re flying a twin, you have to be careful to ensure it’s actually making you safer.

[kqr]

Another example would be something like a leader/follower distributed data storage system. It (and maybe its clients) needs to maintain a coherent view of which the leader node is. This adds significant complexity, and in many cases is no longer worth it.

[jordanb]

Two engines also give you a lot more options for control surface failures. It's objectively safer and why all commercial airliners are (at least) two engine. But it does require more training for the pilot.