Sure, Solo cups are plenty sturdy, but if you've ever tried to pour a hot beverage in one, you'll notice how quickly the plastic starts to weaken. It becomes soft and pliable, because it isn't designed to stand up to much heat. Because Solo cups aren't heat safe and thus aren't microwave friendly, it's safest to keep them away from the microwave. Only heat your food and beverages in microwave-safe containers.
What Can Happen
Solo cups can become very hot in just a short amount of time in your microwave. When you reach in to grab the cup, you can burn your hand on the piping hot plastic. If you microwave the Solo cup for too long, the structure of the cup can weaken as the contents get warmer. Small holes may form, causing liquid to leak out -- or worse, the whole cup might melt after being unattended in the microwave for a long period of time. Not only will you have to clean up the food or beverage mess in your microwave, you'll also have to remove the melted plastic. This is not only challenging, but incredibly dangerous because melted plastic can lead to burns.