Turn up on the wrong day.
If you are standing in the hall talking to the vendor and an item comes through the letterbox, don't grab it as if you own the place already.
Children, don't point to the upstairs toilet and ask if it can be knocked out to enlarge 'your' bedroom.
Don't walk around the property detailing your plans for redevelopment (including a spiral staircase to the loft which would of course be converted) when you just told the vendor that you've dropped the price of your own property due to lack of interest.
If the property details say 'large garden', don't step outside and say in a shocked voice 'the garden is really big'.