[Update: an error in the test program was making it fail and I stupidly typed in the error text before confirming it against the original error. Sorry.]
Reading in an xbm file with the attached image will fail with the message:
GD Warning: EOF before image was complete
gdImageCreateFromXbm() will return NULL.
This does not happen with all images but I was able to reproduce it with two, including one I found in the wild.