I'm going to try to explain this from the manager's point of view. I'm also going to assume that the complaint was something fairly minor, the sort of thing nobody would fire you on the spot for but that might stick in someone's memory. Let's say that you forgot to tell somebody when a particular thing was ready, and they had to ask around to find out it was ready. A small complaint.
For me, when someone does something minor wrong like that, I try to mention it to them right away. But I'm a busy person and I don't always get around to doing that, especially for something minor when I've got a lot on my plate. If too much time goes by before I get a chance to talk to them, bringing it up will make it seem like a bigger thing than it is, so I let it go and just try to remember to react quickly if it ever happens again. Especially if the person is a contractor who is only here for a few more weeks, I'll just let it go. I've got a lot of stuff to do and making a contractor into the ideal employee is optional work: the contractor is doing ok, so I just make a bit of a face and move on.
But then, the contractor applies to be renewed. And I'm tasked with joining the interview. And I remember that time when people weren't notified and that was a problem. So I ask about it. Maybe to you it sounds like I'm complaining about it. Whatever. I bring the subject up. Am I allowed to? Of course. I can bring up any subject I like. What should you do about it? You should react as though you're in a job interview not as though this is a regular day at the office. You have two routes you can take - choose carefully.
Route 1: agree with the interviewer and talk hypothetically about "next time":
There was an incident about a month ago where I was juggling many projects at once. A task was completed on time but I neglected to inform the relevant stakeholders. Unfortunately as you say, they had to ask if it was done or not. The incident embarrassed me and I've developed a checklist to make sure that never happens again. I also have arranged for X to backstop me on this so that if I have to head out for the day before a task is finished, X will send out the announcement that day rather than the stakeholders having to wait until I come in the next morning.
Route 2: argue with the interview and stick up for yourself
It is true that some stakeholders thought I didn't take steps to notify them of a recent task completion. The fact is that X was notified and agreed to pass word on. When I got in the next morning I discovered X had not done what was agreed and as you know I got the word out very quickly once I learned what had happened. I'm sorry you feel that's still an outstanding issue between you and me: I dealt with it by talking to X and didn't feel the need to loop you in on it. I do apologize for that.
You have to be very very careful when telling an interviewer they are wrong. Pretty much the only time it's ok is a case like this where their observation of something that happened may not be complete. Be sure to include an apology for leaving them in that situation of not knowing the whole story. In general I would go for the first approach, though there may be times the second approach will work.