There is a small deviation in the mesh pattern near the upper right-hand corner of the U.S. $1 bill that some people believe to resemble a spider. It being an intentional imagery is very much up for debate, however.
The exact location of the "spider" is near the upper-leftmost corner of the crest-shaped outline surrounding the "1" in the upper-right corner. The deviation is between the crescent-shaped portion of the crest and the rightmost leaf next to it. Two alternate beliefs about this deviation are that it represents an owl or a skull and crossbones (in the style of a pirate flag) when the bill is rotated.