It was recently determined that one of the VC-25s available to the President of the United States was put at risk of a mid-air explosion due to a failure to adhere to proper maintenance procedures for cleaning the oxygen system properly. It was found the oxygen system was contaminated thus running the risk of oxygen reacting with another element resulting in a fire within the system. The aircraft has since been repaired and returned to service.
Note: The article says that Trump was at risk , however the incident occurred in April of 2016 and the aircraft was removed for repairs and inspection shortly after. During this time It was Obama that would have most been at risk if something had happened.