The fact is that there are many things people simply aren't likely to remember. If I asked you right now what you were doing the evening of Wednesday, January 5, would you remember that offhand?
Ah, but what if January 5 was your kid's birthday? Would you remember then?
Coincidences like this come up frequently when someone is being investigated. Shortly after learning about the technique, I came across an example in Alan Bradley's excellent book The Sweetness at the Bottom of the Pie. Not a significant spoiler: an innkeeper being questioned on a guest says he's sure the fellow never stayed there before because the guest's surname is the same as the innkeeper's wife's maiden name, and he'd have remembered seeing it.
Naturally, coincidences are to be handled with care. What makes them work well for this purpose is that they're quick, passing references that basically mean the person being questioned doesn't have to go through her day planner for the last month or whatever. You might think of them as harmless coincidences. The plot does not hinge on them. They just keep things moving.
You could, of course, create a seeming coincidence that's actually meaningful. Say a baker remembers a particular customer because she ordered a birthday cake on the baker's own birthday. The cake may not actually be connected to the baker's birthday, but it could have meaning in the story beyond being a plot-provided memory aid for the baker. Maybe it's a clue. Maybe it's the murder weapon. (Okay, maybe not. Can you drown in cake? Hmm . . .)