Bungled Trade Seen as Showing Weakness of India’s Blocks System

Last week’s big trading blunder, one of India’s worst in years, is exposing a potential weakness in the country’s system for buying and selling large blocks of shares.

In the opening minutes of trading on Thursday, a unit of Avendus Capital Pvt. was seeking to carry out an order to sell as much as 24% of Clean Science & Technology Ltd. for about $300 million but traders thought their initial attempt didn’t go through, prompting them to try again, people familiar with the matter have said. Later, they learned both transactions had been executed, according to the people.

“It ...