Commercial airliners (and other high-altitude craft) are pressurised for a reason -- along with it being a fair bit colder up there, there's less oxygen for us to breathe. Above 10,000 feet (3000m) It doesn't take long for a condition known as hypoxia, or oxygen deprivation, to set in, causing a range of symptoms that replicate intoxication.
In 2008, the pilots of a Kalitta Learjet flying into Ypsilanti, Michigan found themselves suffering from hypoxia and without the help of air traffic control and a nearby plane, the flight would not have ended well -- to put it mildly.
The pilot, who is not named in the video or its description, is clearly heard to be slurring his words, with his speech slow and deliberate. On the other end is Jay McCombs, an air traffic controller for the Cleveland Air Route Traffic Control Center, one of the busiest in the United States. As described on the clip's YouTube page, the pilot's microphone was stuck in the on position, allowing the controller to hear various cockpit alarms:
Over the radio, Jay McCombs tried hard to understand the slow, slurring words muffled among the blaring sirens sounding in the background. The radio was poor, and the pilot difficult to understand, leaving only unintelligible transmission and uncontrollable noise to be heard. ... The events unfolded on July 26, 2008 when McCombs accepted the hand-off of KFS66, which appeared to have a stuck mike creating incomprehensible transmissions. Unclear to those in the Center, however, was that the co-pilots arm was all the while moving violently and uncontrollably on the other end as the captain worked hard to hand fly the aircraft.
It goes on to mention that a nearby pilot was able to help McCombs "translate" what the Kalitta pilot was trying to say and that another controller, Stephanie Bevins, deduced that he was suffering from oxygen deprivation:
Following Bevins initiative, McCombs begins bringing the aircraft to the lowest altitude available in order to alleviate the possible oxygen deprivation. Unable to answer questions, the pilot is only able to respond to direct commands that the controllers now begin to voice. Descend and maintain, they repeat.
Once the plane hit 3350m (11,000 feet), the Kalitta captain recovers almost instantly and his speech essentially normal.