Ignore 404s at your peril
Caching works both ways. Stuff that isn't there once, twice, ten, or twenty times probably won't be there on the 21st try, especially if it's less than ten seconds later!
IP address obstructed to protect the guilty:
a.b.c.d - - [23/Jun/2012:00:33:13 -0700] "GET /apple-touch-icon-precomposed.png HTTP/1.1" 404 314 "-" "Mozilla/5.0 (Linux; Android 4.0.4; Galaxy Nexus Build/IMM76I) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.166 Mobile Safari/535.19" "rachelbythebay.com" "-"
54 hits in 20 minutes. Disgraceful.
Learn about negative responses, whoever you are, and go away.