I’ve just experienced the exact same thing and thus lost money as tickets to an event were purchased for free 🙁
I had reduced the stock available and didn’t realise that this had changed the pricing for the remaining tickets. I too have strange entries in the sales box.
Clearly an issue and one that costs us money, so I’m very unhappy as I can’t now go and tell my customers that they have to pay for tickets that showed up on my website as free.