One of the Ten Commandments is "Thou shalt not bear false witness against thy neighbor" (in the Exodus 20:2–17 and Deuteronomy 5:6–21 versions). Some Christians take it to mean we should never lie. How do they follow this commandment if they understand it this way? If we define "lying" as "saying or writing something that one believes or knows to be false", then it is indeed hard to see how they can follow the commandment (thus understood) all the time.
Just imagine that a little girl, after being teased by some other children as being ugly, asks you, "Am I really that ugly?", and she is in fact very ugly (or at least you do believe that she is very ugly). What would you say? You may try not to answer the question directly by saying something like "Only shallow people pay so much attention to a person's face". But if the girl insists, "Please tell me the truth. Yes or No?" Would you just say "Yes, you are ugly"?
There is a way out. You can try mental reservation. It was invented in the 13th century and further developed in the 17th century by some Catholic priests. The easiest among the techniques is "strict mental reservation": instead of uttering the whole sentence, you utter only part of it and reserve the rest for "mental utterance" (i.e. utter it internally to yourself). According to the doctrine of mental reservation, if the whole sentence is true, you will not be considered by God to be lying even if the part you utter explicitly is false, provided that the false part is uttered for doing good. Applying this to the above example, you could just say to the girl "You are not ugly" and utter internally "if 'ugly' is redefined to mean beautiful". A perfect solution: the girl is happy and you don't have to lie!