Through rounding:
If 2 (both of them) has been rounded down from 2.4, then we are left with 2.4 plus 2.4, which equals 4.8. This would then be rounded up to 5.
Therefore, through rounding, 2+2=5. Or at least it can.
This gave rise to the phrase "2+2=5...for extremely large values of '2'".