There are times when you speak with someone about a mutual experience and remember it differently. "No! That's not how it happened -- this is the story the way I remember it --" And sometimes it is normal to totally forget an event until someone reminds you of it. "Oh! I haven't thought about that in YEARS! " or "It all came back to me in a rush!"
For some multiples these common experiences are exacerbated. A memory may be totally missing even after they are reminded of it -- even to the point where one would swear that it never took place (in spite of coroberating evidence of many people or photos or film footage, etc.).
Multiples may be able to piece together a linear memory for several months then have trouble remembering several months or years. There is much disagreement about whether one can forget something so totally but even a normal person cannot remember what happened each day several years ago. [One exercise I have seen is to remember every day of the 10th grade -- suddenly even normal people have gaps of memory.]