Why phantom spreads happen — and how to spot one before you trade it
A 6% gap that nobody is taking is telling you something. Usually it is a halted market, a dead book or a token that is not the token you think.
The widest number in any scanner is almost never the best trade. Before sizing into a large spread, the question is not "how much can I make" but "why is this still here".
The common causes are boring. The market is halted or in auction on one venue, so its last price is frozen while the other side keeps moving. The book is empty beyond the top level, so the quote is real but two thousand dollars of it is not. Withdrawals are suspended, which means a spot leg cannot be unwound at all. Or the two venues list different assets under the same ticker — a rebranded token, a different chain, a 1000x denomination.
Funding adds its own version: a rate that looks like free money because the contract is being delisted, and the last settlement will never arrive.
The checks are quick. Look at depth, not just the best bid and ask. Compare the mark price against the index on both legs. Check that the contract multiplier and settlement asset match. Arbitrex filters halted and non-positive rows out of the live tables for exactly this reason, but no filter catches every case — the book depth next to the row is there so you can make the last call yourself.