Often occurs when reading email and text messages on the go - you forget about the message until something else in life reminds you and you then remember when/where you received that text or email.
Common belief is that it occurs because since you are not sitting anchored to a desk or a laptop, you forget the message context because your attention is immediately drawn to the car in front of you, your kid screaming in the backseat, having to pay for groceries, or any other activity "on the go". See threadnesia, cell phone amnesia, e-mnesia
Peter: Oh yeah, huh. I think I got that message when I was at the strip club so I kinda forgot about it. Ya know, I do have a wicked case of Digital Amnesia now with this iPhone. Can I get it to you tomorrow?
Lumberg: Yeah, but this is your last warning.
Peter: OK - well just email me
Lumberg (in the Droidzone): Just sent it to you.
Peter: That'd be great!