The hello and goodbye is a routine usually preformed by a girlfriend or wife where she both starts the sexual encounter with a cock in her mouth(hello!) and ends the encounter with the same cock in her mouth(goodbye). (In between that period she is fucked lavishly) Just a standard blow job does not count as a "hello and goodbye".
Situation in which you encounter someone you know - especially someone you have not spoken to in some time - have a conversation then depart, only to immediately encounter the person again, resulting in social awkwardness.
Former neighbours A and B run into each other at the mall and chat for a while, following which:
A. It was nice to run into you!
B. It really was - send my regards to your family.
The definition of this short phrase : If someone says goodbye tension hello pension it means they are a retired dota player. They will usually purchase a mug with the phrase on or an official t-shirt to confirm their retirement.
Rod: Hey there ted , ermm sorry but look what t-shirt i bought ( shows shirt )
Ted : Omg that shirt says goodbye tension hello pension on it !
Friends you never talk to albeit saying "Hello" and "Goodbye" while passing in the halls. These friends are usually people that you have classes with, people you've done a group project with, lab partners, etc.