Never had a source... wait, what?
An article I'm using is about the problems with the Aral Sea. In discussing the paradoxical low salinity of this "closed basin lying in the middle of a country which is completely impoverished in terms of precipitation" the author says: Единственно возможным предположением является то, что Арал некогда имел исток. All of my students translated that as "The only possible explanation (suggestion) is that the Aral never had a source."
There are actually two problems here. One is некогда (nekogda) which looks very much like никогда (nikogda) but actually means "once" rather than "never". The other is the word исток (istok). This is etymologically "from-flow" and usually - almost always - means "source" but in geographic or geologic usage can also mean "outlet". How do you know?
Well, partly you know because the next few paragraphs discuss the Aral's being occasionally connected to the Caspian. In fact, it says
При превышении уровня моря на 4 м от 1905 г оно получит исток в Каспийское море через Аарыкамышкую котловину.Even if you're still thinking "source" (and most of them are), the phrase исток в Каспийское море would have to be "into the Caspian" not "from" it. Partly you know because it makes sense to suppose the Aral is low-salt because it used to drain off into the Caspian. Partly because the article has already referred to "the sea and the rivers flowing into it" so saying it "never had a source" isn't supported by the text. And partly because it just doesn't make sense to say a lake "never" - or indeed "once" - "had a source."
When the sea's level increased by 4 meters in 1905 it gained an outlet into the Caspian Sea via the Sarygamysh basin.
So once again you, as a translator, have to look at a sentence that is seemingly correct and yet cannot be, and then decide where you went wrong. Checking more than one dictionary (in this case perhaps the геология (geology) header in Multitran, or a good scientific dictionary) is called for. Because sense is called for, always. "The lake never had a source" just cannot stand.