The EUR/GBP quote that looked off for a second

[Shadow Trader]

A mini case that shows up in various forms. A trader is watching EUR/USD, GBP/USD and EUR/GBP on one screen. During a fast moment, EUR/GBP prints a price that does not match the other two. Multiply it out and the implied cross sits a few tenths of a pip away from what is shown. It looks like free money: buy the cheap leg, sell the other, collect when it corrects.

The trade gets placed. It fills, the mismatch closes, and the result is a small loss.

The twist is in the fill. The screen showed a mismatch, but each leg was a separate order, filled at a separate moment and on separate sides of the spread. By the time every leg executed, the gap had closed, and the combined spread of three instruments was larger than the gap was ever worth. The edge existed on the display, not in the executable prices.

Also, a lot of those blips are not mispricing at all. They are stale quotes, a slightly slower feed on one pair, or a spread widening that only looks like a price difference. Brokers also use last look and filtering that can reject the one leg that mattered.

Real triangular arbitrage needs very fast, direct market access with tiny costs. For retail, the cleaner reading is that a cross out of line is a hint about where liquidity is thin or where the feed is lagging, not a trade in itself.

It is still worth watching, because the mismatch often shows up just before a fast move starts on the lagging pair.

Has anyone logged cross mismatches against the legs, and did any of them actually survive the spread?