The synoptic gospels (Matthew, Mark, and Luke) agree that Jesus was crucified alongside two other criminals (see e.g. Mark 15:27). They disagree, however, on whether they both taunted Jesus. Here is Mark 15:31-32:
Matthew 27:44 agrees with Mark. By contrast, this is Luke 23:39-43:
It certainly seems that Luke has altered the story which he found in Mark. The depiction of the repentant thief in Luke comes at exactly the same point as the depiction of the unrepentant thieves in Mark and Matthew. In all three synoptic gospels, we have just been told that the priests mocked Jesus, saying "why can the King of the Jews not save himself?" or words to that effect. In all three synoptic gospels, the next sentence (after the thieves) tells us that darkness came over the land, from noon till three.
This is hardly conclusive though - it's not impossible that Luke was aware of a different aspect of the story and chose to go against Mark for that reason. Such alterations do not have to be seen as "making things up". The Apologetics Press suggests that the man at first reviled Jesus, but later repented. If Luke was aware of this, it would explain the discrepancy and there would be no errors here.
(Or he might have made it up. You decide.)