Check the "use-by" date on the packet. Patties are made of mince, which is more exposed to the air than whole meat, so goes off more quickly. Also, although frozen foods usually make them last longer, there are some foods such as meat which go off while their frozen state does not change their appearance.
In other words, if you eat meat that's older than a few days, it can go off very quickly in the stomach, especially mince or patties. Preservatives and colorings and flavorings are often used in commercial meat products to disguise this. If you have eaten patties, note whether you feel lacking in energy, cloudy-headed, easily tired or unwell the next day.
Be careful when selecting patties at your grocery shop. In many countries they are made out of *any* meat, including cats or dogs. The only way to be certain of the meat you're getting is to buy it fresh (not frozen or packaged) from a reputable butcher.