It seems to me that there's a difference between:
1) not remembering which checkout line you used, and
2) imagining that you lost sight of someone , and then a police dispatch told you to look for that person, and then you move your car, and then you spot him walking between two houses, and then he comes back and circles your car and runs off again, and then the dispatcher repeatedly asks you for an address, and then you get out of your car to look for an address, and then the guy who ran off jumped at you while spitting corny movie dialogue.
That sounds reasonable, but might not be. The problem, from the rememberer's point of view, is that no one tells you beforehand that the next ten minutes are going to be seriously significant and you ought to pay attention. It's only afterwards that any of what happened becomes important.
I have been the victim of a serious crime. I was able to identify the gun used, but when asked what happened shortly before the newsworthy stuff, I simply couldn't accurately recreate it. What I found was that I filled in using my own intentions and general heuristics about what constituted normal. I was told by police that remembering the gun pointed at you is much more common than the face of the person holding it, just because of the emotional connection with getting shot -- I guess we tend to focus on where the danger seems to be.
I can't say in GZ's case, but I would grant some leeway.