Originally a genuine report of a life-threatening fault. Now used humorously to report any kind of problem.
John Swigert, Jr. and James Lovell who, with Fred Haise Jr., made up the crew of the US's Apollo 13 moon flight used (almost) this phrase to report a major technical problem back to their Houston base.
Swigert: 'Okay, Houston, we've had a problem here.'
Duke: 'This is Houston. Say again please.'
Lovell: 'Houston, we've had a problem. We've had a main B bus undervolt.'