Indian benchmark indices registered their worst single-day fall in more than eight months on Friday as concerns related to the review of the double taxation avoidance agreement or DTAA with Mauritius came back to haunt investors. Fear of a sell-off by foreign investors as the R hit a four-month low tamped down sentiment further. The 30-share benchmark Sensex, which started the day above 17,000 points, dipped below that psychological mark as index heavyweights across sectors were dumped by institutional investors wary of the fact that their income could come under the tax scanner.