Hospital emergency departments get a lot of infected sores that are blamed on spider bites. In reality, most of the "bites" are probably just nasty bacterial infections. In most cases, the spider is nowhere to be found. (Would you stick around to be squished?)
Not having the spider makes it hard to positively identify a spider bite. In the United States, the brown recluse spider is often blamed for bites, but the brown recluse only lives in a few states in the southeast part of the country. There are poisonous spiders related to the recluse living in other areas, but they are not nearly as dangerous as their Dixieland cousins.
Hobo spiders and black widows get blamed nearly as often as the brown recluse.
Doctors are almost as guilty as patients for incorrectly diagnosing ugly skin ulcers as spider bites. These pictures all show sores that the patient -- and in most cases, the doctor -- thought were spider bites.