I have always ridden on "whatever tires where laying around", and had pretty good luck with them in the past. Well though a series of parts/bike swaps I ended up with a set of 26x1.5" Specialized Nimbus Armadillos for my drop-bar mountain bike conversion.
On the way to get some groceries earlier, I ran over this chunk of metal:
The entire portion of the skinny part was stuck in my tire, as you can see it was about the length of my thumbnail:
I heard the thumping on the ground so I stopped to see what I ran over and noticed that the tire was still holding air in it. I first thought about leaving it in there and limping home, thinking that it sealed into the tube just enough to get me back home. I finally decided to just rip it out (took a bit of tugging) and replace the tube so I could finish my grocery run.
Surprisingly, when I finally got it out, I didn't hear any hissing from the tire going flat!! I don't think I will be risking crap "free" tires again...unless I get another set of these free again, haha