I noticed on IB that one particular mini futures contract price is lower than the full contract? Normally they are identical price quotes. But lately they are off by a bit. Can one arbitrage that? Buy N number of mini-contracts and sell short the big contract?
Trade frequency in mini futures is usually much lower than in the full contracts. Sometimes there might be no trade for several minutes. If you look at the LAST PRICE for such an instrument, there might be a huge divergence from the price of the regular contract, because the the last price for the regular is for example from 2 seconds ago, but the price for the mini future is for example from 14 minutes ago because nobody traded that contract since then. If you want to know what prices you can really get right now, in this very moment, you have to look at the bid/ask quotes for both instruments. Another possibility is that you are looking at different contract months, for example still September for the mini but maybe the full contract already switched to December.