In field metering situations you can have the timeline of each meter, when it records data points, and then you have the different timeline of the backend service, when it actually _receives_ data points.
Bonus points if the meters can send data out of order.
I did a tender for aggregating events and emitting notifications from a fleet of transport vehicles that reported metrics over LTE.
When connected they would emit events as they happened, but when there was poor service or disconnection they would buffer them and send them as a burst once they reconnected... in reverse order.
I forget the finer details, but the service I was writing was supposed to notify of trucks entering and leaving geofences, inactivity etc. anything you can think of where the order of events is important.
Bonus points if the meters can send data out of order.