Posting the evidence here would go a long way. Firstly, it would let others see the evidence and comment. It might be a case of misinterpretation, perhaps, for example, the paytable shows the payout as "coins" rather than "x line-bet", so that it may only be 500x line bet, but shows as several thousand "coins". Indeed, MG do this with their Cashapillar game by showing it is possible to win "2,000,000 coins". This is actually only a miserable 2000x total stake, because the claim only applies to all 100 lines being played at max coins, and getting the "Cashapillars" all lined up to produce 100x the 2,000 x line bet jackpot (or 20,000 coins @ 10 coins per line max bet).
Some software (RTG) has an overall limit for feature payouts buried in the rules, which is 40,000x line bet, but turns out to be a mere 2000x total bet on a standard 20 line slot.
Once such explanations can be ruled out, then some form of cheating can be investigated.
Posting the name of the software & game involved would be enough for others to investigate. No need to post the name of the casino unless it has bespoke software, and this is often a bad sign in any case, although there are striking examples where bespoke software means the complete opposite of "suspected dodgy casino", one example being 3Dice, and an earlier example being the Lasseters/Ausvegas group before they brought Orbis in.