Imagine if they did immediately refute that this occurred. Wouldn't you be more suspicious of the defensiveness?
A: five years ago you made a pretty bad mistake that hurt me personally.
B: I must have let my priorities slip, and it sounds like in my oversight I hurt you personally. Reach out with more details and we can make this right.
or
A: five years ago you made a pretty bad mistake that hurt me personally.
B: I don't really make mistakes like that.