Carole Blake lists 29 different ways to screw up the process of submitting something to an agent. The list is both informative and entertaining in the exasperation that sneaks through at times. Some of the activities she lists are mindboggling, but my favourite is the most tragic:
I once received a large parcel that weighed almost nothing. Inside was a rubbish bin and a letter saying the writer assumed the submission would end up there so was sending me one to speed up the process. The partial for a crime novel that was attached looked rather good. I left the bin, letter & ms on my desk. Next morning our office cleaner had removed the contents and put the rubbish bin neatly next to my desk. There was no way to contact the author despite a story on our website and some tweets … That was the end of that.
There are 28 other things to avoid at the original article here, as well as a letter sent to Carole raging against the damnable cabal of editors who refuse to accept the author's submissions: http://www.bang2write.com/2013/03/29-ways-not-to-submit-to-an-agent-by-carole-blake.html