In striker-fired handguns like the Glocks, some Sigs and others, there are some cases in which the handgun is unintentionally discharged in the holster. The cause in almost every case has been identified as something catching the trigger as the gun is inserted into the holster. Sometimes this is the corner of a shirt or jacket, a drawstring cord-lock or a strap.
The striker-fired handgun's trigger pull weight is lighter than a true double-action, but heavier than most single-action firearms. The nature of the pull weight is that the handgun can be fired without putting a lot of pressure on the trigger and so if one holsters the gun in a well-fitting holster and something fouls the trigger, the extra resistance may not be noticed and the handgun can be fired as it is inserted, often resulting in injury.
This has happened enough with Glocks (one of the first striker-fired handguns to be extensively used) that there is an injury called "Glock Leg", where the user shoots themselves in the leg while holstering, but Glocks are not the only ones this can happen with.
As long as one is careful to make sure the holster is clear and nothing enters the trigger guard while holstering, the gun will not fire.
Because carrying concealed often puts the holster in a difficult place to see well (behind the strong-side hip), many people holster the pistol off-body, then place the holstered handgun on the belt. This way, one can be SURE nothing will foul the trigger and if it did, the handgun is pointed in a safe(r) direction.
