I once heard this anecdote about a Russian teacher, who shall remain nameless. He was in a cafeteria and wanted to ask whether they had a tray, поднос (podnos). What he actually ended up asking was whether they had понос (ponos), or “diarrhoea”.
One major problem when you’re trying to learn Russian is figuring out which syllables are supposed to be stressed. “To write” is писать (pisat’), stressed on the final syllable. If you stress the first one… well, you can probably guess what that means.
This is to say nothing of the dreaded tone languages, like Chinese, where pitch variation (rising, falling and so forth) is used to distinguish between words. For example, look up “ma” in a Chinese-English dictionary and you will find meanings as different as “mother”, “hemp” and “horse”.
And then you have the occasions where it comes down to life and death. The Old Testament (not particularly recommended reading for the squeamish) contains an account, in Judges 12:5-6, of a fatal language test. The men of Gilead, having inflicted a military defeat on the Ephraimites, identified fleeing enemies by demanding that they say “Shibboleth” (שבלת), which they would mispronounce as “Sibboleth” (סבלת). Some forty-two thousand were said to have been killed as a result.
Language is a powerful weapon, not least when it backfires and explodes in your face.