This Bible contradiction is from the Skeptic's Annotated Bible.
There were 120 believers after the ascension.
Acts 1:15 View context
And in those days Peter stood up in the midst of the disciples, and said, (the number of names together were about an hundred and twenty,)
There were more than 500 believers before the ascension.
1 Corinthians 15:6 View context
After that, he was seen of above five hundred brethren at once; of whom the greater part remain unto this present, but some are fallen asleep.