This has been discussed extensively in this thread, but I realize it's hard to keep track of everything that gets posted, so I will summarize the problems.
For starters, they trashed the cottage before they were finished collecting evidence. The premises were reasonably tidy, outside of Meredith's room, in photos taken on November 2 and 3, but when the next set of photos was taken, on December 18, the place was a shambles.
http://www.friendsofamanda.org/bed_nov_02_07.jpg
http://www.friendsofamanda.org/bed_dec_18_07.jpg
http://www.friendsofamanda.org/kitchen_nov_02_07.jpg
http://www.friendsofamanda.org/kitchen_dec_18_07.jpg
Also see the pdf on the IIP site:
http://www.injusticeinperugia.org/Meredith_Kercher_murder_reconstruction_graphic_-_Ron_Hendry.pdf
It explains how they threw bloody boots under the bed, and then concluded that the resulting bloodstains were part of the post-crime staging. If that isn't bungling, I don't know what is.
Here are some guidelines for handling DNA evidence, put out by the US Dept. of Justice:
- Wear gloves. Change them often.
- Use disposable instruments or clean them thoroughly before and after handling each sample.
- Avoid touching the area where you believe DNA may exist.
- Avoid talking, sneezing, and coughing over evidence.
- Avoid touching your face, nose, and mouth when collecting and packaging evidence.
- Air-dry evidence thoroughly before packaging.
- Put evidence into new paper bags or envelopes, not into plastic bags. Do not use staples.
http://www.ncjrs.gov/txtfiles1/nij/bc000614.txt
Notice the condition of the gloves in this closeup from the December 18 kitchen photo:
http://www.friendsofamanda.org/kitchen_dec_18_07_closeup_of_gloves.jpg
Obviously someone handled a lot of stuff with the same pair of gloves, and this is how evidence becomes contaminated. Over the course of several hours of video taken at the crime scene on Nov. 2-3 and on Dec. 18, no one is ever seen changing into a fresh pair of gloves. This is significant, because the video shows sloppy evidence gathering, like this:
http://www.friendsofamanda.org/stefanoni_swabbing.jpg
Notice how she rubs the bloodstain, then reverses the tip of the swab and bears down on the bloody spot with her thumb. Unless she changes gloves, this procedure is almost certain to cause DNA transfer between samples, which is why the guidelines listed above tell investigators to change gloves or use a clean instrument before handling each new sample.
Now take a look at this video clip showing another sample being collected:
http://www.friendsofamanda.org/dropped_swab.avi
She drops the swab on the floor, then picks it up and resumes her wiping of the bloodstain as though nothing had happened.
These are just a few examples among many documented by the police themselves, in their photos and video.