Wrong estimates of trading activity, market size and market share
Even to this day, a lot of people misconstrue the turnover numbers declared by the exchanges. This often leads to wrong estimates of trading activity and market size & share. So if A buys stock X for Rs 1lk & B sells for Rs 1lk, the exchange turnover = Rs 1lk & not Rs 2lks.
To calculate market size estimates based on exchange turnovers, the right way would be to multiply the exchange turnover by 2 and then calculate the %. More on turnover in the below post.