One of the rituals of hajj is to stone the three pillars at Jamaraat by first picking pebbles at a place called Mina. As we collect the stones, we think about our bad habits that we want to hit away and get rid of to become closer to Allah.
Imagine if you had to collect a stone for every wrong action you've done, would you remember each one? In the same way we forget about some of the wrong actions we've done because we think they are small but it doesn't matter what the size of the wrong action is, Allah sees and knows everything. In Hajj we have the chance to think about our actions and ask Allah to forgive us.