There are many products available that contain gases and mixtures of gases stored in a pressurized steel container. You are probably most familiar with propane tanks that are used for cooking or grilling. These steel tanks may seem empty when you can no longer get a flame from them, but they often have small amounts of pressurized gas remaining.
If you deposit one of these tanks in the main recycling stream, it will get mixed in with everything else, contaminating the recycling stream. If the tank is accidentally compacted in a sorting facility, it could explode, potentially injuring workers and damaging the facility! Never put containers that held flammable gas in your municipal recycling stream.