When Callan is in the pub saying sorry to Lonely the glass in Lonely's hand is empty, the next shot it is full.
Two references are made to the capability of a Noguchi Magnum to fire a round through plate steel. However, when Callan first shoots the handgun at the Section's firing range, there is only a stack of telephone directories behind the targets to stop the bullets fired. In fact there is a solid wall behind the phone books, which are probably there to stop bullets ricocheting from the wall, not to stop the bullets in the first place.
As Callan opens Schneider's wall-safe, the action jiggles the picture hanging above it, which it clearly would not if the wall were real.
The criminal record in Callan's file gives his forenames as David Edward; his personal card above it seems to read David Arthur.
The events of the film refer to the attempt on Schneider's life as taking place on Sunday 22nd October, which would date the film to 1972. The tax disc on his car, however, is dated Nov 73. This is also verified by a newsstand reference to Royal Wedding pictures - this was the marriage of Princess Anne to Captain Mark Phillips, also in November 1973.
When Callan and Lonely climb through the skylight the wall of the chimney outside shakes.
When Callan is trying to slip Hunter's surveillance team he stops to 'give directions' to an old lady in an alleyway. As he do so, a member of the public (an older man with glasses) attempts to walk down the steps towards them but is pulled back out of shot, presumably by a member of the crew.