I got an email his morning that began "Just a quick reminder..." about a report that I was supposed to have submitted last week. I get a lot of emails like this, and I send a lot of them too, usually in the summer to people on my softball team - "Just wanted to remind you that I haven't been paid for registration..." - or during the rest of the year to my students - "Just wanted to let you know that there was no document attached to your email..."
There's something very passive-aggressive about the "just". Ostensibly, it's supposed to indicate that you're not angry or annoyed - the email is "just" about this or that, even if the reader might have immediately assumed otherwise. But it's never "just" about the topic and the fact that the sender has to deny being annoyed right off the bat is usually a good indication s/he is, in fact, even more annoyed than you initially thought.
The only worse and even more passive-aggressive opening to this kind of email? "Just a friendly reminder..."