Why does Oracle flip plans?

As most of us will be aware, every SQL query has an explain plan that lists out the steps that Oracle is going to take in order to execute the query and it helps us a lot in tuning the query too. But at times, the plan that executed well for a query today gets flipped and generates another bad plan tomorrow thereby making the query run longer.

What do you think might be the reason for these flips? Does it mean we can't trust Oracle's explain plan generating capability or are there any other circumstances on which this behavior hinges?