The purpose of this study was to investigate action slips during acquisition of a new skill. Whereas most studies on slips have collected action errors of adults using the diary method, we observed a young child’s daily performance using video recordings. One child’s mouse rinsing activities after toothbrushing were recorded for one year from age four to five, and analyzed in terms of action slips as well as microslips—miniature versions of slips. Slips were repeatedly observed when a child took a cup and filled it with water. Quarterly analysis reveals that the total number of slips did not decrease monotonously: whereas two types of slips, commissions and adjustments, decreased over one year, other types increased from Q1 to Q3, then decreased in Q4. This suggests that in the process of the skill acquisition some types of slip may fade out, but others may rise or last out.