On that day, Flitter learned three very important lessons.  
One: Miss Derpy’s habit of changing clothes behind unlocked doors remains unchanged.  
Two: Being an easily concerned sort, Miss Derpy will give chase if you take off in a dead run without warning. And she is alarmingly fast, all things considered.  
Three: Miss Derpy is exceptionally clumsy.
 
Coincidentally, Cloudchaser also learned three valuable lessons.  
First: Location  
Second: Location  
Third: Location