The US/UK Gallon Trap
This is the single most common volume conversion mistake: a US gallon and a UK (imperial) gallon are genuinely different sizes — a US gallon is about 3.785 liters, while a UK gallon is about 4.546 liters, roughly 20% larger. Using the wrong one silently produces a result that's off by a real, meaningful margin, not a rounding error, and the ambiguity is exactly why "gallon" alone (without specifying US or UK) is an incomplete unit for any calculation where precision matters.
This US/UK divergence isn't limited to gallons — pints, quarts, and fluid ounces all have different US and UK definitions too, inherited from the two countries' historically separate standardization of imperial measures after they diverged, which is why a UK recipe and a US recipe using "cups" or "pints" as a unit aren't actually interchangeable without conversion, despite using identical unit names.
Metric volume units (liters, milliliters) don't have this ambiguity problem at all — a liter is a liter everywhere, which is exactly the kind of international consistency the metric system was designed to guarantee, and precisely why scientific and most international contexts default to metric volume units specifically to sidestep the US/UK imperial divergence entirely.