Things run smoothly most of the time, but once in a while there is an issue with an engine, forcing a flight to land prematurely as a precaution. This is never a threat to safety as long as pilots are trained to do just that. It happens about 2-3 times per month and considering the enormous amount of engines out there running continuously, the problem is statistically non-existent.
More often there is an engine-related problem forcing a emergency landing because people has difficulty breathing due to smoke. In most planes the cabin is pressurized by bleed air from the high pressure pneumatic systems in the engines. If there is a leak there might be smoke entering the cabin, sometimes to a degree that people will need medical attention after landing. In recent constructions there are alternative ways to pressurize the cabin and this kind of mishap may eventually be history. An early clever solution was the 4 cabin compressors in the nose of the DC-8, (also history), providing guaranteed smoke-free cabin pressure. The air intakes for those compressors were slanted on both sides and the saying went; how do you recognize a DC-8. It’s the only airline that smiles back at you.
A couple of times a month people run out of runway and end up more or less bent in the terrain beyond the end of the paved area. As magnificent a B747 may be as an aircraft, its off-road capabilities are utterly disappointing. There is some safety concerns here, but normally the nuisance for the passengers is limited to not being taken to the terminal other than by bus. To the cockpit crew on the other hand it’s a matter of great embarrassment since it normally involves som mis-calculations or not so fancy flying. The fact that half of the worlds runways are to short, preventing aircraft to take-off and land at their maximum structual weight, and preventing airlines maximum profit from their billion-dollar investments, will be dealt with in an other article. It’s one of the dumbest things in the industry, not quite as dumb as tax-free sales before a flight rather than after landing, but close.