I have a Mead batch that has the exact thing you are describing. It has been aging for over a year, and still tastes bad. It's difficult to describe in a more quantifiable way, but after a year, I don't expect it to get better. It's likely going down the drain next time I start a batch, as I need the bottles...I strongly suspect an infection of some type, as it was my first and only attempt at fermenting in a soda keg. I thought I had cleaned and sanitized it adequately, but there are a lot of places for bugs to hide in a soda keg, and I may not have gotten them all.
As far as it being safe to drink? Yes. No known harmful pathogens can survive in even a weak alcohol solution for any period of time. I don't have hard numbers, but I have spoken to a Biology prof where I work, and they confirmed that home brew is safe to drink, even when infected with a wild bacteria. It may taste terrible, but the infection isn't harmful to you.
As for sanitation vs. cleaning et all, everyone has thoughts, and experiences of what has worked for them. I prefer to err on the side of caution, and scrub the hell outta everything with PBW, and hit it with StarSan, just to make sure. However, I have a friend who simply rinses his fermenter, and caps it with tinfoil till it's time to start another batch, and he has great results.