The wick. On a cold commute, e-liquid moves slower, so the wick may not re-saturate between puffs. If you chain-vape—say, three or four long drags in a row—you can outpace the liquid. The mesh coil heats up, the wick dries, and you get that harsh, burnt taste.

Another common cause: leaving your device in a warm pocket. Heat thins the liquid, but it also makes it flow too fast, flooding the coil. Then you get a gurgle and spitback, not a burnt taste. That's a different issue, but both come down to temperature swings.

The fix for a dry wick is simple: stop. Put the device down for five to ten minutes. Let the wick soak. Then take a gentle, short puff—not a long haul. If the taste is still burnt, the wick is scorched, and that's not fixable at home.