On the last project I was working on, I built a backend on Node.js v4 for an online course site. For a long time I was trying to convince our team leader to switch to Node v6, since it supported ES6 and I couldn't wait to use the new JavaScript features like, e.g. classes. However, he was always reluctant to make the switch, since there were other priorities at the time.

At some point, I found out that inserting 'use strict' at the beginning of each Node.js module, enabled the experimental ES6 (harmony) features in Node v4. Needless to say, I was super excited and immediately started using classes and other ES6 goodies everywhere, even refactoring already existing modules.

Shorty after that, we noticed that our servers were leaking memory and started crashing almost every day. At the time, I had no idea what the problem was - and believe me I tried everything to find a solution - until a couple of months later we switched to Node v6, and everything miraculously returned back to normal. In the meantime though, during those 2 dreadful months between v4 and v6, we had to setup cron to restart our servers every single day at 04:00...

Never use experimental features.