I am writing this thread in response to my experience with Oculus support. Oculus support has an extremely quick first line support, however they did not have the tools to deal with this issue, and the second tier seem to go through this dance of "please provide pictures of the issue", which seems like a waste of time. I am writing this thread both to let Oculus know their routines on this issue should be improved, and to help those who anxiously await the support they want.
Fear not! Most of the time, the Rift isn't faulty!
There is a calibration tool for the IMU (Inertia Measuring Unit - it's the part that measures how the Rift is rotated, tilted etc) that I used with great success. I want to link to the guide I used, and point out some important parts about this:
1) YOU'RE DOING THIS AT YOUR OWN RISK
This is important. It seems like Oculus do want some sort of control of who uses the tool since it's not that easy to get. It would be pretty easy for the Oculus team to pack this into a nice GUI with steps of how to perform a recalibration, so since they haven't done that, it could indicate that in some cases, this might not be the right thing to do. Without the support team saying "yes, you can run this tool", you should not assume that this will fix your problem, nor be certain that this won't make things worse. At worst, it can be harmful to use this tool, so be cautious when using it, and do this at your own risk
2) This is specifically for issues where the horizon line is slanted or tilted. I think the best way to notice this is just to put on the HMD, pull up a grid view (like the universal menu). If you notice it here, it most likely is there. This rules out the surface you put it down on being slanted in itself, etc. This is never due to a slanted sensor, as the sensor knows what is up and down.
It is very important that the calibration is run when the Rift has been completely disconnected for hours and hours. Leave it alone overnight. This is because the tool calibrates the IMU at all temperatures, since temperature affects the IMU. It runs for ten minutes so it can go from cold to hot and calibrate all the steps along the way. It is also important that it is placed on the most stable surface you can find.
Follow all the steps and use this guide, you can download the tool there:http://www.vrheads.com/how-fix-oculus-rift-calibrations-problems
This fixed my problem. I hope this can help others. Remember that this might be the right cause of action. Contact Oculus support if you are experiencing this issue, perhaps even say that you do intend to use the calibration tool, and ask if it's the right thing to do. I hope this post can make Oculus' response to this issue be more efficient, as it is such a well-known issue that there should be a standardized response.