Could be a bug but the more likely explanation is that the field doesn't update until you try to leave it. So you put in a wrong date first, try to leave the field, get error message, then type in valid date and save a pic before leaving the field.
Ding ding, this is the likely answer. I've seen this exact issue before on multiple sites and services, and it's always been found that the form expects 18+.
There was nothing in the post indicating what app this was and "Remind" is a generic enough word, even if upper-cased, to make the service not obvious. It could be a porn-site for what we know, in which case that date should naturally be rejected.
I'm going to guess they typed 02/17/2008 the first time to denote Feb 17th as is fairly standard with American apps. It then told him it was invalid because there arent 17 months in a year because it was the wrong format. They typed it correctly after and took a screen shot before pressing submit. (Making it so the error was still on screen from the previous bad submission. )
Another possible case no one seems to have mentioned. That the CSS doesn't do that kind of spacing automatically, and that the user manually put in spaces this creating an invalid date for the lulz.