It is easy for someone with perfect hearing to hear incorrectly when words are confused because of similarly sounding letters. The scriptorium -- the ancient "copy center" -- worked by having one person dictate to a group of scribes who produced the copied manuscripts. Even when a scribe copied a manuscript alone, he would have read a portion out loud and then written it down. During the time from reading a text to writing it down, errors are bound to happen. Writing down something that sounds the same as that which was read is a common error that is detectable.
There are a group of vowels and diphthongs (vowel combination) that came to be pronounced alike. This kind of error is referred to as an itacism when one of the seven is confused with another. In the following examples, notice the underlined letters in the Greek, which are the itacisms, and then see how it affects the English translation.
A strange reading is found in codex Sinaiticus and Codex Alexandrinus which
was caused by an itacism. 
Certain consonants also were interchanged because they sounded alike. In this case, the closing letter of one form and the opening letter of the next (k & s) when combined form the sound of the single letter (as in x). It can be represented as "ek sou" or "ex ou".
This is another instance in which ending sounds can become combined with the beginning sounds of the next form. Here a closing consonant could also be the opening consonant of the next form. A scribe would not be able to hear the difference.