The big concert starts at 9:30 PM, a strange time for any concert, let alone a standard classical one.
The opening drive into Chicago begins southbound on Lake Shore Drive, then northbound on LaSalle (recognizable from the Batman films), then south on Wabash, then suddenly across the river, coming east on Washington, where the car finally stops at the Lyric Opera building. The interiors were shot elsewhere, so nothing looks like the actual hall.
When Tom walks off the stage and hides from the spectators, the laser pointer of Clem's rifle should not be able to point at him.
There are numerous 'plot holes' that can be realized in the film's story.
The assistant clearly slashed Ashley's throat (with a big shard of glass). Yet, later when her body's found, there appears to be nothing on her neck.