rephlex writes: The USB controller used in the Broadcom BCM2835 (which is the SoC the Raspberry Pi uses) has buggy drivers which have been causing problems for many of its users. In addition to this the Pi can only supply an unusually low amount of current to its USB devices, just 140 mA approximately, and using a powered hub to sidestep this limit exacerbates the issues caused by the USB drivers. Even Ethernet is affected as the Ethernet controller used on the Raspberry Pi is connected to the SoC via USB. This has resulted in packet loss and even total loss of network connectivity in certain situations, see https://github.com/raspberrypi/linux/issues/29. Attempts have been made in the past to fix the buggy USB drivers as there are other devices which use this problematic controller. None of these attempts seem to have achieved very much.
rephlex writes: "There are multiple Raspberry Pi users on the official Troubleshooting forum experiencing hardware problems with their Pi. It appears the Raspberry Pi has problems with USB, specifically with powering peripherals that require 140 mA or more. That situation causes a voltage drop that results in the USB voltage becoming out of specification. This has resulted in malfunctions with USB devices such as Wi-Fi dongles, even when connected to the Pi via a powered hub. Also, there appears to be Ethernet problems caused by this "excessive" (but well within USB specifications) current draw: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=6&t=6037"