The problem is that the program is a fairly simple python script that uses some 'command line' programs to do all it's heavy lifting; curl and growl-notify. To invoke these, I was using os.system() to invoke the program, building up a big command-line as the parameter. As the programs take parameters which are coming from the data being downloaded, I was trying to be careful to escape the data when constructing the command lines.
Unfortunately, I'm an idiot, and didn't understand the escaping rules. In particular, I wasn't taking into account backquote substitution.
So, I was really suprised one day, when I got a message displayed in growl that was humongous; and the tail end of of the message was a wad of html. Going to the twitter site, to see the message there, I immediately realized what happened. The twitter message included some text like this:
... `curl http://www.yahoo.com`
Got that fixed up pretty quickly, by avoiding the use of os.system() and using an os.spawn() variant instead, which (hopefully) avoids the shell completely.
BTW, I continue to use my twit-growl to get 'popups' for incoming tweets, and generally use the wonderful tweetbar Firefox extension for posting, and reviewing tweets en masse.