A client called telling me he got an HTTP error when trying to upload an image to use in a product post on his WordPress/ShopperPress site. Then he could no longer access any pages on his website.
The image file in question had an apostrophe in it (Men’s). The apostrophe, or maybe it was a single quote, tripped a mod_security rule on the server (Apache) which blocked all traffic from his IP address.
The server admin removed the block, I removed the apostrophe from the file name, it uploaded fine, and all was well again.