The reason Codex Bezae is argued about is its text of Acts, which runs roughly a tenth longer than the version in every other early manuscript. The additions are not padding. They supply names for people the standard text leaves anonymous, motives for actions it leaves unexplained, and circumstantial detail of a kind that reads like someone who knew more, or like someone who wanted the story to be fuller. Where the standard text says the disciples argued, Bezae can say who argued and why.
Two readings of that have been on the table since the seventeenth century. One holds that these are old, that an early and independent line of transmission preserved material the mainstream lost, and that the shorter text is the abbreviation rather than the longer text the expansion. The other holds that a second-century editor with a taste for clarity worked the book over, and that Bezae preserves his edition faithfully. The manuscript cannot settle the question, because it is the only substantial Greek witness to what it contains, which is exactly what makes it valuable and exactly what makes it hard to test.
Theodore Beza, who gave the codex to Cambridge in 1581, was himself wary of its readings and said so in the letter that accompanied it. It is an unusual thing for a donor to write, and four centuries of scholarship have not resolved the doubt he registered.