There has just been an interesting study of climbing accidents caused by poor belaying techniques carried out by a large swiss climbing wall.
Perhaps surprisingly their conclusion was that the vast majority of the accidents were caused by habitual, bad belaying technique and not so much by a lack of attention or distractions.
They also identified some of the sloppy habits that caused the majority of the accidents. There is a full(ish), translated report at Rock Climbing Company
A lot of the accidents were caused by belaying with an Italian Hitch, but I left this section out for the time being because not many UK climbers use this technique.
A lot of people may find it surprising that a fair proportion (6 out of 15) of the serious accidents ( leaders hitting the ground) invoved Grigris - although it should be pointed out that every case involved incorrect use of the device.