I was blending frozen fruit when the blade unit wobbled free. It sent chunks flying and made a grinding sound that scared me. The base threads had stripped from me cranking it too tight last time. Always hand-tighten only and give it a test spin before you plug it in.
Was it a homemade smoothie with fresh fruit? That happens when the fruit starts to break down and builds up gas. I learned to always drink mine within a few hours or refrigerate it immediately. Using less ripe fruit can help if you need to store it longer. Also, avoid filling the container all the way to leave some space.