This shortcut has cost me so much over the years. Heaven forbid you're typing out a long comment and accidental hit it when the focus is off the text box. Now you hit forward and your text is gone. It wasn't until fairly recently that the browsers kept text box content under these circumstances. In the past you'd just lose it all. Hell, maybe IE still does, but I know Chrome keeps it. So many long winded Metafilter and Slashdot posts lost to the bit bucket...
Other times, focus detection breaks randomly and you're in a text box but something happens and, whoops, off you go to the previous page. I don't know how often these early web guys thought we'd be going back, but mapping it to one of the largest and most frequently used keys is asinine.
Andreeson recently said that his team always thought of the front and back buttons as half-assed solutions until they could figure out something better. Guess that's probably never going to change.