The history of language contact is the history of thinking that what other people said was the answer to the question that you asked.
"Thus were immortalised in generations of atlases such geographical oddities as Just A Mountain, I Don't Know, What? and, of course, Your Finger You Fool." ― Terry Pratchett, The Light Fantastic
Here are a few borrowed words showing that whoever appropriated it didn't really understand the language they got it from...
Word: alcohol
From: Arabic
To: English
Problem: al is Arabic for "the". So alcohol is al kuhul, the kohl (see Etymonline). Similarly al-kimiya alchemy, al-jabr, algebra, al-qaliy, alkali...
Word: apron
From: English
To: English
Problem: The word when we got it from French was napron as in a napron. A little bit of misunderstanding of word boundaries later, and we have an apron.
Word: cherry
From: French
To: English
Problem: The English assumed that cheris was cherries - making a single fruit a cherry. But in French, a cherry was un cherise. The singular happens to end in an 's'.
Word: pea
From: Middle English
To: English
Problem: There existed a word pease, which you may be familiar with from nursery rhymes. (E.g. Pease pudding hot, pease pudding cold, pease pudding in the pot five days old.) It was both singular and plural - like sheep - but it sounds like a plural. So now we have the singular pea.
Word: vitabu
Meaning: books
From: Arabic
To: Swahili
Problem: When 'kitab', book, was borrowed from Arabic, it was assumed that 'ki' wasn't part of the meaning - it was the singular 'ki' found in lots of other Swahili words: Kiswahili (Swahili), kidole (finger), kitu (thing)...
Now the plural of 'ki' words is 'vi': vidole (fingers), vitu (things). So the plural of 'kitabu' must be 'vitabu'...
If you find these kind of histories interesting, Language Log has a fine collection of folk etymologies, egg corns and mondegreens.