I've had a few weird things happen with my K750i over the past few months. The first is a hardware fault - it's joystick is on the way out, and it keeps thinking I want to press 'enter' when I'm actually moving the cursor around. Very frustrating, and it feels like it'll only be a matter of weeks or maybe months before the thing gives out entirely. However I've managed to solve a couple of minor annoyances, which I'll mention here in case it helps anyone.
First, is that my messaging memory kept filling up incredibly quickly. Initially, I seemed to have a lot of space, but it was slowly reducing despite the fact that I used MyPhoneExplorer regularly to clear out Sent items / drafts and incoming messages. Well, MyPhoneExplorer doesn't show you picture messages. Somehow I ended up manually deleting pictures from my inbox, but the ones in my Sent folder were piling up. They're all gone from the phone now, and I have space again! Yay!
While clearing this space out, I found the solution to my next minor annoyance. I've never been able to find where to turn on cell site information on the K750i. I didn't think too hard about this until recently where I needed to know which site I was connecting to. Every phone I've ever had supported this, so I knew it'd be there somewhere but was unable to find it in any of the phone settings areas. Well, it's stored in Messaging Settings! Which makes... sense? So from the main menu, select Messaging / Settings / Area Info.
So there you have it, I've solved all my questions about the phone just in time to throw it away.