Why with a BTST trade you may end up in trouble?

i have read that with BTST trade we may end up in trouble ,but it does once in a way especially when you trade B group and illiq-uid stocks. why is it so and why B group only?

The problem is with liquidity, check this post for a detailed explanation - http://zerodha.com/z-connect/queries/stock-and-fo-queries/consequences-of-short-delivery-nse-bse