The one that makes me crazy is dreaming that I am snoring (because I am in real life), but not waking up and the snoring part becomes a vivid part of the dream.
The one that really freaks me out is waking up with hypnopompic sleep paralysis. I have experienced it off and on since I was a kid. Fairly normal in about 10% of people, but still a weird experience.
Hypnopompic Sleep Paralysis
During sleep, your body alternates between REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. One cycle of REM and NREM sleep lasts about 90 minutes. NREM sleep occurs first and takes up to 75% of your overall sleep time. During NREM sleep, your body relaxes and restores itself. At the end of NREM, your sleep shifts to REM. Your muscles are "turned off" during REM sleep. If you become aware before the REM cycle has finished, you may feel that you cannot move or speak despite seeing your surroundings. This happens because the chemicals released by your body to shut down muscle movement (so you don’t physically act out your dreams) haven’t cleared before consciousness begins. Slight eye opening captures your surroundings in the conscious mind and your brain incorporates the image into the REM dream state making a hybrid experience between reality and dream. You believe you are acting in the real World but are still only acting in the dream state vividly enhanced with real World imagery woven in.