Mumbai: An unusual spike in Bankex options marked Thursday's expiry of BSE derivatives contracts, sending traders scrambling during the newly introduced Closing Auction Session (CAS).Most notably, the Bankex 65,000 Put expiring on Thursday surged from ₹6.65 to ₹987, a jump of nearly 14,700% within minutes. The move came as indicative closing levels during the auction briefly pointed to a near 3% decline in the Sensex, sharply steeper than its fall before the session began. The index was down roughly 0.3-0.4% just before the closing auction session began.ET BureauBankex slides 1.67%, Sensex slips 0.70%; Bank Nifty falls 0.47%, Nifty loses 0.48%Traders said the sharp drop in indicative closing levels sparked panic among derivatives traders, triggering stop-losses and adding to the volatility.Several other Bankex put options around the 65,000 strike also witnessed sharp spikes, with premiums jumping between 500% and 4,500%. Social media was abuzz with chatter that several options sellers suffered heavy losses because of the unexpected expiry-day volatility.Read more: NSE IPO: Sebi approval likely soon, says chairman Tuhin Kanta PandeyAt the close, the Sensex ended at 76,933.59, down 539.35 points, or 0.70%, while the Bankex settled at 64,313.15, down 1,094.16 points, or 1.67%. The declines in NSE's comparable indices were much smaller, with the Nifty closing at 24,090.85, down 116.90 points, or 0.48%, and the Bank Nifty ending at 57,509.95, down 273.80 points, or 0.47%.