You might be surprised how common that is when someone is really excited about doing something for the first time. No matter how much they plan, prepare, rehearse... the moment comes and their mind just blanks.
It’s similar to freeze response, stage fright, & test anxiety.
“I have no idea what I just did!” ... It’s a kind of disassociation.
Merged with honesty. Most people just sort of smile nervously and nod and hope no one can read the OMFG! What just happened??? Scrolling being their eyeballs.
While people occasionally do something totally freakish & bizarre whilst dissociated due to nerves? When it can be double checked people find they usually did exactly as they’d planned. Even if they have zip zero nada zilch recollection of it, or just the rushing blood in their ears, cold sweats, and swirling words on the page.
Ahhhh adrenaline.
For the record, I have almost zero recollection of my wedding. I’m pretty sure I said “yes” or there would have been hoopla, but I could have been stood before Darth Vader marrying a shrimp fork, for all I can actually remember of it.