Mark (John Mark) quit the first missionary journey in the history of the Christian people (Acts 13:13). In retrospect, this journey turned out to be one of the greatest events in history, and Mark made the horrendous mistake of quitting.
However, Mark became the writer of the second Gospel, a canonized saint, and one of the greatest people in history. Mark fell, but he got up. Better yet, he stayed up. It's not good enough to get up after we fall. We must ask for God's grace to correct the reason for the fall, so we don't fall again.
Mark quit the first missionary journey after seeing Paul call for God to strike Elymas blind (Acts 13:11). Mark may have been shaken by the intensity of the spiritual warfare that occurred. However, Mark must have corrected his problem, since he later wrote of expelling demons, handling serpents, and drinking poisons without harm (Mk 16:17-18).
Mark may have quit the first missionary journey because God raised up Paul to lead this journey, although Mark's cousin, Barnabas, was originally the leader (cf Acts 13:1, 2, 7 and 13:13). Mark may have needed to learn to submit to his elders and to clothe himself with humility (1 Pt 5:5). He learned to "bow humbly under God's mighty hand, so that in due time" (1 Pt 5:6) God lifted Mark up to write one of the Gospels.
If you've fallen, get up and stay up by correcting the reason why you fell.