The soldier hit with a rock falls over and starts firing his Sterling SMG. Of course he shouldn't have had his finger on the trigger but that's merely a failing of the character. The major issue is the seemingly endless stream of shots fired from the 34 round magazine.
An early scene, a Humber Pig APC has a flat tire seems to be a major tactical error. A vehicle would not be on it's own in a hostile environment. If anything goes wrong the vehicle is stranded along with all the troops. Which is exactly the scenario in this sequence. There should be at least one more vehicle capable of hauling a disabled vehicle to safety.
At the end of the initial shooting scene, in Belfast, a train goes past in the background. It is clearly a dark blue British Rail DMU with the white double arrow symbol of BR. But Northern Ireland Railways (NIR) was not part of British Rail in 1975. NIR DMUs at the time would have been two-tone red and white or blue and white.
The uniformed police sergeant contacted by DI Hollis when he is attempting to enter the Houses of Parliament is wearing the 1918 War Medal, more than 50 years after it was awarded.