Someone who constantly receives and sends text messages in conversation, making them difficult to talk to. In some situations this is considered reasonably antisocial behavior.
Jake: Hang on, I just got another text.
James: Your such a texthead. Put your phone away and join in with the REAL conversation.
When two people, or groups of people, meet up unexpectedly and instantly hide their true feelings about each other under awkward and blatantly fake politeness.
After just leaving an unwanted conversation and joining another with a genuine friend:
Newcomer: God, I thought that would never end.
Friend: You two have seriously plastic manners. Just admit you don't like talking to one another. It's sad watching you two stumble through a conversation.
Stating the obvious in self defense after making a really stupid mistake.
Person having lost their car keys:
Well, its hard to find something when you don't know where it is.
Person listening: Good Nobservation.