Plot hole: Necros had absolutely no way of knowing that Bond and Saunders had arranged to meet at the café 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 café doors (or had planned this ahead of time - unlikely), and set the trap up well in advance of him 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 Ritchie appears at a pizza party sponsored by a local radio station, his manager Bob Keane is handing out copies of Ritchie's first album. In reality, the album had not been released until April 1959, two months after Ritchie's death. As the actual LP includes his biggest hit "Donna", Ritchie had not yet recorded (or even composed) the song in this part of the movie until a few scenes later.