Plot hole: Necros had absolutely no way of knowing that Bond and Saunders had arranged to meet at the cafe when they did, and wouldn't have had anywhere near enough time to track them down and set up his elaborate booby trap. Saunders only suggested the meeting place and time a few hours earlier, and it was kept strictly between him and Bond. The scene in Tangier where Whittaker tells Necros to kill another British agent takes place on the same day Bond arrived in Vienna; meaning that Necros got from Tangier to Vienna (a 5 hour-plus flight), tracked down Saunders, acquired the materials to booby trap the cafe doors (or had planned this ahead of time - unlikely) and set the trap up well in advance of he and Bond getting to the fairground. There was nowhere near enough time for all of that to happen without Necros having psychic knowledge of Saunders' movements.
Plot hole: When Bond uses the laser on his Aston Martin to cut the police car in half, the laser supposedly cut all the way through the length of the bottom part of the car. If that's true, the brake lines would have also been cut. The top half of the car slides off when the driver applies the brakes. If the laser cut clean enough for the top half to slide off, then the brake lines were indeed cut by the laser. How could the driver apply the brakes if the brake lines were cut?