When a cell has too much of one thing, homeostasis gets rid of some of it and balances the cell out. Just like when you have too much stress, you find ways (coping mechanisms) to get rid of some of it.
The type of mutation that causes this translation error is called deletion. Deletion occurs when a section of a DNA is deleted or lost. In cystic fibrosis, three nucleotides are deleted that results in the lost of phenylalanine.