to tell cyclists

[HappySeven]

that if you lose your chain, drop down a gear, keep pedalling (gently) and it will usually get itself back on.

No more oily hands: it's revolutionised my commute?

Why didn't someone tell me years ago?

If your chain's coming off regularly, you might want to check your gears are indexed properly; it could be that when you're shifting, it's pushing the chain over slightly too far. HTH!

It's just been serviced so I think it's just user-error, but thanks!

If it's just been serviced then they may have replaced the gear cable. A new cable often "beds in" and/or stretches a little soon after it is fitted, which can cause problems like the dérailleur pushing the chain over too far. Your gear cable probably needs tightening a little after the recent service, and/or the end stops adjusting.