Lemons and limes are toxic to dogs. The substance psoralen is contained in various citrus fruits and can have adverse effects if consumed by a dog or cat.
It can cause vomiting, diarrhea and sensitivity to light, among other unpleasant symptoms, and is good reason not to feed your dog lemon (although most dogs don't enjoy the sour taste anyway).
Despite this, there are plenty of videos on YouTube showing owners that disregard this risk and feed their citrus fruits nonetheless.
This is what happens:
