Check your wheel spokes!

This is my friend's bike. He hasn't had it long, and it recently developed a death-wobble. The day before filming this, I tightened the spokes on the front wheel and trued it. This was filmed at work: he had ordered a new rear tire and this was all in the process of putting his new rear tire on. I didn't do anything with the rear wheel/tire the day before because we knew his rear tire had come in and was waiting for him at the shop.
Wheel spokes should be checked and tightened at least every oil change, more often if you do a lot of long-distance riding or spend a lot of time on the highway at high speeds. Most people don't know that, and my friend didn't know that either.