[yters]

If they knew the labels were switched, they know which one held caffeinated coffee...even the physicist is too sophisticated...

[sukilot]

They don't know if the labels were correct before, or even if the tanks were swithet.

[xdavidliu]

that's only if it was switched an odd number of times. It could totally have been one of those shell game things where it's switched a couple dozen times, and no one knows whether it was even or odd.