Weird cursor line bug.
 

I have this somewhat weird bug where my mouse cursor just turns into a line and then fixes itself after a couple of minutes. What is that? What causes that?

It may not be a bug. I think you need to characterize that a bit more. What's open? Does it happen based on particular mouse location? Further, what window manager are you using? And are you in an application at the time like an editor, debugger, command line utility? Odds are that your efforts to analyze what's happening enough to give sufficient info for someone to help you may lead you to self-resolve this question.

Sounds like you're seeing it go from a pointing arrow to an underline for some unknown reason. My recommendation is still that you look to see if you're in an editor, or if an editor is open; but you're not focusing on that particularly right now.

I have a strange one that comes up from Firefox and GMail; like if my mouse is hovering over a folder name for some short period of time, it will raise like a help text or tooltip, repeating that folder name. And if I use a key sequence to swap my workspace, that tooltip may stay up and be on top of a totally different workspace, where I have no open browser window at all. It's rare, but happens and my only remedy is to go back to my workspace with that browser hover over and move away to cause the tooltip to disappear, then I'm fine. More annoying really.

I suspect it's something similar to this.

I use xfce and my cursor just changes to a long line. and it stays like that until it changes back.

When you log out, does it take "forever" for Xfce to shut down (and go to run level 3, where you use startx or even to a GUI log in)? For some reason, Xfce looses it's mind and does crazy things every now and again (and I don't know whey and I'm not sure than anybody actually does know).

What I've learned to do is
if I've had Xfce running for a few days (or maybe even for a couple of weeks); I don't log out very often (naughty me, but it's my home system) and have experienced weird problems with Xrfce -- like on restart the cursor disappears altogether or it turns into the standard X cursor or some other nonsense. Blowing away the ~/.cache directory (it's automatically recreated) seems to put a stop to that sort of thing.

Might be worth a shot.

Hope this helps some.