I've had unexpected reminders pop-up now and again, but always put it down to me having pressed OK too many times when browsing the EPG and changing channels. I've found it quite easy to unintentionally set a reminder this way, especially when you change channel to watch live programming but the guide is showing you future scheduling.
Also, we don't really make proper use of the "user" accounts in my house and tend to just randomly "log-in" to these, and use "guest" most often. So, even if you did want a reminder it doesn't appear as the user that set it hasn't logged in on the day the programme is actually on.