How NOT to get a $30k bill from Firebase

This meant that if there were 100 donations, the application would make a call to the database once but the actual act of reading would equate to 100 times twice — once for the values required to calculate the total and second time for the number of supporters — totaling 200 read request. When the number of supporters started to hit the thousands and page views climbed to more than 2 million, it became very easy for read requests to the database to reach billions. The Colombian startup would have faced database connections maxed out issues or something similar even if they went with a traditional cloud hosted solution instead of Firebase.

Source: medium.com