I booked three people on the high speed train from Venice to Florence. I put in the correct date but in looking at the options, the website changed the date to the next day without me noticing - perhaps because I scrolled through all of the available options on our travel date? I inadvertently purchased tickets for the next day, not the day I wanted. A post on Tripadvisor suggested this could also be due to not setting the hour of departure but I did do that initially and the train I ended up booking was the exact same train for the following day. Here is the other thread: https://www.tripadvisor.ca/ShowTopic-g187768-i20-k7237837-Trenitalia_Ticket_Change_Problem-Italy.html
Trenitalia's online purchase system does not have a preview screen that allows you to double-check your booking before clicking to purchase. It takes you straight to a credit card screen with no other information about your booking and you have to click yes to buy or no to cancel without being able to double check your purchase. I was confident in the information I'd put in so I clicked yes. It did not occur to me that the website would change the date on its own. (If you get this screen as I did and haven't yet double-checked, I recommend cancelling and starting again since the dates and times you put in may not stick.)
We've already got hotels booked so changing our plans was not an option. I thought there must be a way to modify the reservation online, especially since I realized the error immediately. I could not find any way to do this on Trenitalia's website without paying a 32.50 euro fee as with any reservation change. The customer service line was closed so I made the online change and paid the 32.50 euro and immediately emailed Trenitalia with my reservation code asking if the change fee could be reversed. I got an email back the next day telling me to call the customer service line.
I called the line and got through after a couple of attempts. The agent told me that if I'd called before paying the change fee, they could have fixed it but since I paid the change fee, there was nothing they could do. Hopefully posting about my mistake will save others a few euro.