Lemon meringue can curdle if the lemon filling is overheated or not cooked enough, causing the eggs to curdle and separate from the lemon juice. To prevent curdling, make sure to cook the lemon filling over medium heat and stir constantly until it thickens and comes to a boil. Remove from heat as soon as it reaches a boil, and strain it through a fine-mesh sieve to remove any lumps or cooked egg.