Be careful in Taksim square and Istiklal street (Turkish: İstiklâl Caddesi; then main walking street of Istanbul with lots of shops and restaurants and at night street performers).
Maybe several years ago only Aksaray area in Istanbul had a bad reputation for unsafe bars where they charge you with 500 TL (about 200 US$) for a drink with a nasty ugly girl just sitting beside you. The menu of these places shows 500 TL for single wine drink !!!!.
But nowadays Taksim area has also become unsafe with similar old tricks. Usually there are guys hanging out on Taksim Sq. or on Istiklal St. and they approach you with this trick that looking at the girls are free and you do not need pay anything. When questioned, they claim that these bars are legal and bad things happen only in Aksaray, not here in prestigious area like Taksim. But they are lying. Bad things are happening in Taksin as well. For your safety, please do not even look at these guys when they address you on the street and do not talk with them.
For example, there is a bar in one of the alleys of Istiklal St. on the right hand (when heading down the street from Taksim Sq.) with a neon sign of "DEEP" club. Do not go to this place. There is a narrow staircase going up and there you see 4-5 girls dancing. There is a sister club one or two alleys away with the similar manager and same trick. In this bar the staircase goes down and with the same story. This will happen in a bad memory from your trip to Istanbul and maybe 1000 TL charge for two drinks or bad fight and similar problems which ruin your travel. Hope to be useful.
I saw this to happen about 1 month ago in March 2015