If the principal does not exist or has no capacity.
If he expressly or impliedly personal liability. The third party may when contracting with an agent, create a condition that the agent should be personally liable on the contract, and if the agent agrees he will be personally liable for any breach of contract.
If the custom of that particular trade makes him liable as in the case of del credere agent.
If he signs a negotiable instrument in his own name without making clear, on the face of the document that he is signing as an agent.
If he executes a deed in his own name, or while purpoting to act as an agent when he is actually acting on his own behalf, or if there is no principal in existence.
If the agent acts for a concealed principal, if he so desires. A concealed of the principal use the principal if he desires. A concealed principal is one whose existence or identity is not