pulvo

One vowel from another word, and they are relatives. Pulvo is gunpowder; polvo at rank 1,608 is dust. Say them carelessly and you have said the wrong one, because Esperanto vowels are the whole difference and there is no context to rescue you: both are fine, dry stuff that gets everywhere. Worse, this is not a coincidence to file away as a trap - the two words are the same Latin word, pulvis, arriving twice by different roads. Esperanto gave the direct borrowing to explosives and the Romance-flavoured one to dust, and now a learner has to keep them apart on a single letter.

And a third neighbour. Pulvoro is powder in the general sense, the fine grains of anything: dentopulvoro, bakpulvoro. So the family runs pulvo (what fires a gun), pulvoro (powder of any kind), polvo (dust that settles on furniture). Three near-identical shapes, three clean meanings, and no way to guess which is which - this is a set to learn as a set.

The vocabulary it lives in. Pulvo turns up in adventure stories, in history, and in the proverb-like phrase li ne inventis la pulvon, he did not invent gunpowder, which is exactly the English insult about not being the sharpest tool. Around it sit pafi to shoot, kanono, armilo a weapon, and eksplodi to explode. Pulvujo is a powder flask, built with the same -uj- container suffix that Esperantujo stretches so far in the other direction.