Mind the gap!
Most flaws scribes encountered when they corrected a manuscript could and would be fixed immediately. Others, however, required some time. The scribe may have wanted to deliberate the text with someone else or look something up, for example in a glossary. Such time-consuming fixes were usually done after the text was fully copied out. In such cases the scribe would leave room in the text to reinsert the checked reading at a later stage.
When he had finished copying, a text could potentially have been riddled with lacunae, standing at the ready to receive the new readings. Scribes would hide any and all traces of these corrections by using the same ink color and writing in the same manner, which means they are difficult to spot. The practice of using lacunae for inserting improved readings is particularly common in autograph copies. Translators, for example, often needed some time to find equivalents for unusual words in the original language.