What could happen if two incompatible lubricants are mixed?

Incompatibility between two lubricants can lead to a physical and chemical reaction between the base oils (they can form two phases or degrade completely) and/or a chemical reaction between the additives (possibly creating deposits).

In both cases, the lubricant degrades. This can lead to faster wear, and even to the equipment breaking.

Related questions:

• Can two lubricants be mixed together in the same engine?
• Can I simply refill the lubricant tank or do I have to change the lubricant?
• What is mixed lubrication?