To the extent feasible, waste should be segregated and not combined.  Mixing of different type waste poses dangers:

Incinerated lab hood at the  University of Kentucky (below) following an explosion involving the mixing of two incompatible wastes.  The explosion threw glass shards over 20 feet into the lab. Fortunately, no one was injured.

In 2008 a GT student received acid burns to the face after accidentally pouring sulfuric acid waste into a solvent waste container.