Aside from the fact that Mayella and her father brought false charges against Tom in the first place; and that Mayella lied through her teeth during most of her testimony, she made several other mistakes while on the witness stand. First, she contradicted herself at one point, changing her story back and forth. When Atticus asked her if Tom had beaten her, Mayella said
"No, I don't recollect if he hit me. I mean yes I do, he hit me."
"Was your last sentence your answer?"
"Huh? Yes, he hit--I just don't remember... It happened so quick."
Secondly, Mayella let it slip--although not verbally--that her father wasn't always "tollable" towards her. When Atticus asked if Bob was "good to her," she responded,
"He does tollable, 'cept when--"
"Except when nothin'," said Mayella. "I said he does tollable."
"... Except when he's drinking?" asked Atticus so gently that Mayella nodded.
At another point, during Tom's testimony, he remembers that
"She says she never kissed a grown man before an' she might as well kiss a nigger. She says what her papa do to her don't count."
It can be assumed from Tom's statement that Bob and Mayella may well have had more than just a normal father-and-daughter relationship. Bob's and Mayella's version of what happened that day varies wildly from Tom's testimony, so the reader must decide who is telling the truth. Most readers agree with Scout's and the author's point of view--that Tom was truthful and that the Ewells were liars.