[X-Istence]

Tap and hold the message you sent over iMessage. Click send as sms.

[mbreese]

Which is a totally undiscoverable feature.

[dhimes]

Not only is it undiscoverable, you don't even think you need to do it. Like others said- you take out your phone, send a quick message, put your phone away and do something, only to find out your message wasn't sent. Possibly because the recipient, unknown to you, didn't have data access.

[Zombieball]

Undiscoverable features is the new way to design apps! Look at Snapchat

[danudey]

'Swipe randomly in random directions from random screens until you see something you didn't know was there' is the new 'simple and direct'.