Excuse or not, panel two is absolutely true. You might think it won't happen to you, then BAM!

Ketchup EVERYWHERE.

Justin makes a very good point in this comic. Change blindness is a real thing, but it's not guaranteed to work, and there are plenty of x-factors that could result in someone noticing. Heck, Justin knowing what Grace is up to should make it easier for him to notice (but fortunately for Grace, I find it more amusing if he doesn't notice the changes).

That said, we all agreed that this is non-canon (anyone who says they didn't agree to that is clearly lying), so while I would normally agree with Justin, there's really no long term consequences to worry about.