Cubs fans know Mark Grace as #17. But his first Topps card shows a very young Mark Grace
wearing a scrub number, #53. This is strictly a spring training number. When he came to the majors in May, 1988, he wore #28 for his first three games. The Cubs were on the road at the time and I'd guess they didn't have many numbers available. When they returned home, Grace got #17, which he wore for the first time on May 6. He would wear that number the rest of his career.
Here is Ronnie Cedeno wearing a scrub number, #68. Most ballplayers prefer to not share a number with an offensive lineman.
Cedeno wore #5 for most of his time with the Cubs. Based on his performance, it would have been OK for him to continue to wear #68. Luckily, his time with the Cubs was fairly brief and good 'ol E6 is currently the Mets' problem.