When an explosion crippled Apollo 13’s command module, the crew used the lunar module as a “lifeboat.” The two modules had been built by different contractors, so when the mission was over Grumman sent a tongue-in-cheek bill to Rockwell for “towing” the ship to the moon and back:

The Associated Press reported that “North American Rockwell replied that the invoice had been examined by the company’s auditor, who pointed out that North American Rockwell had not yet received payment for ferrying LMs to the moon on previous missions.”

(Thanks, Perry.)