When I call a web service I'm usually spitting out what I get back by logging it to the history list. And sometimes my workflow will hang with an error message along the lines of:
Retrying last request. Next attempt scheduled in less than one minute. Details of last request: HTTP InternalServerError to ...
Invalid text value.
A text field contains invalid data. Please check the value and try again.
If you see this, it usually means you're logging something to the history list that's greater than 256 characters.