I want to put in my experience in case it can help anyone who is currently overstaying a tourist visa in Europe. Of course, the best way to avoid any of this is to simply be aware of the rules, sometimes it can be confusing or it can slip your mind, putting you in a tough situation. For starters, as of 2017, you can stay in the Schengen zone for 90 days out of a 180 day period. Here was my situation:
I accidentally overstayed for about 2 months when I was working (under the table) in Spain, as I somehow thought that i could stay in Schengen for 180 days total instead of 90. In a slight panic, I went on some forums, asked my spanish friends, and they almost all told me the same thing, if you are in Spain, nobody really cares if you are an overstay at least when you are American. I was still paranoid about this, and was a bit nervous when i got to the airport for my departure flight to Seattle.
The passport checker at the airport literally did not even look at my passport before stamping it and handing it back to me. And after that I was on my way.
In conclusion, all the research I did at the time, plus my own experience has told me that if you have overstayed and want to leave, the authorities in Spain and Portugal will more than likely not bat an eye about it. I have heard of course that if you are leaving from The Netherlands or Germany or possibly Scandanavia, they are much more strict, and you will more than likely face some sort of penalty or at the least some questioning.
Important note: I have not been back to Europe since, so I am unsure if there are any consequences waiting for me, but as I mentioned, this is mostly for people who are already overstaying and are weighing their options. If you are trying to leave without being hassled, I would recommend Spain or Portugal.
Hope this helps,