I was fishing the Thames one year in the spring. Standing on a particularly steep, muddy spot on the bank and hooked into something. It felt big but not really fighting hard. Just going side to side in the current. I finally get it in close to the bank but it was too far under in the dirty water and all I could see was a big dark shape and it was kind of hung up on some roots or something. So I gave a good hard pull and it let go of the roots and this big, dark round thing came lunging out of the water right at my feet. What I saw (or thought I saw) was a giant snapping turtle charging out of the water to attack me! I started scrambling backwards in the mud and then started slipping and was sliding down toward the thing and was just about to start a full scale, screaming panic when I realized what it really was.
A green plastic garbage can lid. All covered in algae and mud. The 'head' was the handle, all jammed full of mud on the side.