In many cases if you see a price discrepancy in your quotes by the time you try to arbitrage it it simply vanishes as if it never existed. There are a few reasons for this. One is when the quoted price you're seeing is wrong or is simply untradeable. Another possible reason is a bid-offer spread that wasn't accounted for. And in some cases, it is simply because the model you are using is wrong, or there is some risk factor you haven't taken account of.