The freaking post office. I mailed two keyboards last week at the same time, one to San Francisco, the other to San Diego. The former reaches its destination in two days. The latter shows no movement from the main sorting center until just now, where it got an arrival scan...in Dallas. Fortunately the buyer has been extremely nice about it, but still.
Also, another keyboard I sent to someone much closer (Michigan), has been delayed for no good reason. Heck, I could have personally delivered it to him in a fraction of the time it's taken.
I can understand mail getting delayed in New England because of the weather there, but there's no reason for a package labeled as 2-3 day shipping should be taking more than a week to get to California.