"Profitable in the Strategy Tester, losing money live" usually gets blamed on slippage. One specific, mechanically checkable cause is worth ruling out first — and it's documented behavior, not a rare edge case.
This one isn't from a client audit — it's a platform mechanic every Pine Script strategy that pulls in a higher timeframe should be checked against, because it's easy to introduce by accident and TradingView's own documentation explicitly warns about it. If your backtest and your live or paper results have quietly stopped agreeing and you've already ruled out fees and slippage, this is the next thing to check.
What request.security() actually does
request.security() pulls a value computed on a different timeframe (or symbol) into your script's current context — the standard way to check, say, the 1-hour trend from a 15-minute chart. It takes a lookahead parameter that controls how it fills in values on historical bars. As of the current Pine Script version, its documented default is barmerge.lookahead_off — meaning out of the box, it does not leak information from the future into the past.
The bug shows up when someone explicitly sets lookahead=barmerge.lookahead_on. People usually do this on purpose, not by accident — it removes what looks like an annoying one-bar lag on the higher-timeframe value, and plenty of forum answers recommend it for exactly that reason. TradingView's own docs carry a direct warning about this setting:
