I didn't notice I was using the offline mode during a market and lost hundreds of dollars in sales. I was completely unaware. Now I know. My first few sales went through. I thought I was good to go for the day and online. To answer a question I read, no, Square doesn't notify you that there are pending offline payments that need to be "moved" from the terminal to the internet/square. No warning at all. I found out over a month later because I missed the 24-hour cut-off. But remember, I didn't know they were offline sales, nor did I see anything warning me that they were offline when I added up the sales from my terminal. I had to calculate my sales and give 20% to the market. It was a fundraiser. So there you go. Oh, and there is no way to contact the customer(s). I was told what I need to do is have a book in which I write down the name, phone number, and last 4 digits of the card to keep in case of this happening or any problems. I was hoping they would offer some fee processing fees to help out a small business to offset my loss, but it was a no. I REALLY think a warning email text or message or even the sales being in red so they stand out would be nice. It seems doable. I truly had no idea. I didn't realize the mode was turned on and an option (my bad?), so I turned OFF the offline mode. That way, I have to turn it on if the internet is spotty manually. Not happy.
