After climbing the 'waterfall' in the catacombs with a pickaxe still in her hand, after the shot cuts, she has no pickaxe anymore, then in the next scene she has it again.
The film claims there are 7 million bodies in the Catacombs; however, there is an estimated 6 million according to the official site of the Catacombs.
After the main character hits her head and awakens alone, she finds the gate that has been chained and locked on the outside. The yellow police tape is on the inside. However, it's not inconceivable that the gate was sealed by the police, and they could've placed the warning tape before locking the gate.
The police are apparently competent enough to run out an entire rave and seal up the catacombs, but incompetent to not even find someone lying unconscious in the tunnels everyone was running through.
The appearance of the police at the rave causes everyone to run away, including, apparently, the police, who are seen in the stampede of people through the tunnels.