This is a variant rule present in the Dungeons Master's Guide.
A character might drink one potion while still under the effects of another, or pour several potions into a single container. The strange ingredients used in creating potions can result in unpredictable interactions.
When a character mixes two potions together, you can roll on the Potion Miscibility table. If more than two are combined, roll again for each subsequent potion, combining the results. Unless the effects are immediately obvious, reveal them only when they become evident.