Life is unpredictable most of the time, and so we all bail out on plans when we cannot avoid circumstances. Flaker friends are always unreliable ones because they never prioritize you. Their scattered priorities mean that they can never remember important dates, commitments, and pre-decided events.
There is always a lame excuse or mostly even no-excuse for forgotten dates and missed commitments. Perhaps it is their preoccupation with social media, the bad habit of attending your calls in between conversation with others, and mismanagement of their time that makes them forget important details. Whatever the reason may be, flakers that are passive-aggressive and manipulative are never worthy friends.