XTB shares slipped 5% today (Wednesday) after the Polish Financial Supervision Authority (KNF) upheld a PLN 20 million (about $5.35 million) fine. The sanction covers customer checks, product targeting, conflict controls and CFD risk disclosures.Even after Wednesday's decline, XTB was up about 143% this year and only 6.7% below its August intraday record. The selloff also came as XTB announced that its shares will join the STOXX Europe 600.KNF Ends Its Administrative ReviewThe KNF issued the final decision on August 28 and published it on Tuesday. It left in place the penalty first imposed on March 30 after XTB asked the regulator to reconsider the case.The regulator said XTB did not properly determine whether customers had enough knowledge and experience to understand the risks of the services and financial instruments offered to them. That finding covers January 2022 through August 16, 2023.For a period ending September 17, 2023, KNF also found shortcomings in how XTB defined target markets for financial products. It cited possible conflicts tied to XTB's client-facing HOT list.[#highlighted-links#]The regulator also said information given to clients or potential clients about instruments and CFD risks was unreliable or misleading.FinanceMagnates.com reported the original PLN 20 million decision in April. XTB recorded the amount as a one-time expense in its first-quarter accounts, so the final ruling does not create a new, previously undisclosed charge.In April, XTB said it had already revised the registration form and onboarding mechanism in line with KNF guidance. XTB then requested reconsideration. It had not posted an updated response to the final decision on its investor-relations site by Wednesday afternoon.The case has implications beyond one penalty. KNF is reviewing how domestic and cross-border firms sell CFDs to Polish retail clients, with a focus on how brokers assess experience, knowledge and risk awareness.Index Entry Fails to Stop Wednesday's DeclineXTB said its shares will enter the STOXX Europe 600 on September 21. Index inclusion can bring demand from funds and exchange-traded funds that track the benchmark, although XTB did not estimate the potential flows."Inclusion in the STOXX Europe 600 is a significant milestone for XTB," Chief Executive Officer Omar Arnaout said.The positive announcement did not prevent the shares from falling against a rising Warsaw market. At PLN 174.24, the stock was about 5.4% below its record closing price of PLN 184.16 set on August 31 and 6.7% below the PLN 186.70 intraday high reached on August 28.The pullback is small beside the 2026 rally. XTB shares had jumped 8.6% after preliminary second-quarter profit beat market expectations in July.That performance remains heavily tied to derivatives activity. CFDs generated 96% of XTB's first-half gross result, even as stocks, ETFs and investment plans drew customers to XTB.Volatile markets have also supported other listed brokers. CMC Markets reported record client assets and raised its annual income outlook in July, although its business mix differs from XTB's.One Stock Nearly Matched 200 TradesXTB's earlier rally produced an unusual result in Bankier.pl's seven-week Wakacje na Gieldzie investment contest.The winner made nearly 200 transactions across Polish stocks and crypto-linked products, finishing with a 47.73% return. The two runners-up took a simpler route: both bought only XTB shares on the first day and held them through the end.The second-place entrant bought at PLN 126 in four transactions and earned 45.56%. The third-place investor accumulated the stock between PLN 125.90 and PLN 128.00 and finished with a 44.58% return.This article was written by Damian Chmiel at www.financemagnates.com.