Lemon bars can crack two ways: if you over-beat the eggs when mixing and added too much air to the batter, the air will escape in the heat of the oven. Just mix the eggs in until they’re fully incorporated, but resist the urge to beat until foamy. Two, lemon bars can crack from drastic temperature changes. Upon removing them from the hot oven, place them near the oven on a cooling rack to gently cool. Never take hot custard and place it in a fridge immediately after cooking.