One piece of advice that's often given to novice cooks is to season dishes with a light hand, because it's easier to add flavours than to take them out. In practice, even experienced cooks may find themselves needing to compensate for a little too much of one flavour or another. Lemons are an ingredient that can dominate the flavour of a dish if too much is added during cooking. If you mistakenly put too much lemon in your dish, its overpowering flavour can be toned down by adding a complementary or contrasting element.
Increase the quantity of a dish, when possible, to dilute the impact of the lemon. If you are making a lemon-based sauce for Chinese food, for example, make a half-batch without lemon and combine it with your original batch. Freeze the leftover quantity for another meal.
Add a pinch of salt to sweet dishes or desserts that contain too much lemon, and then add enough sugar or other sweetener to take the edge off the lemon's acidity. The pinch of salt allows you to use less sugar than you otherwise would.
Shake or grind salt and pepper into savoury dishes with too much lemon until the flavours come back into balance. Salt is a potent enough flavouring to interfere with the brain's ability to perceive the lemon's acidity. A pinch of sugar will also help.
Drizzle hot sauce onto a savoury dish or add fresh or dried chilli peppers. Chilli heat and sour acidity balance each other, which is why they are often combined. Add small pinches of salt and sugar to balance the flavours further.
Hot sauces vary widely in flavour, with some fairly salty and others fairly acidic. In this instance, it's best to use one that is salty but has little acidity.