Often when I play TF2 (usually as spy/scout, but sometimes as other classes), I'll find myself suddenly [blasted/incinerated/insert your own favorite way to die] by something I never saw coming. Many times there's no obvious error on my part that caused this, but just plain bad luck (bumping into someone around a corner while invisible, finding out as scout that there's a sentry behind that corner you just passed, etc.).
Although some of these could be avoided by playing more cautiously and paying greater attention to my surroundings, that takes time that I do not have when in dangerous situations. (You can't take 360 spins to ensure nobody's sneaking up on you, you have to kill your target and get out of danger.) Is there any real solution to this, or do I just have to take the deaths, curse my luck, and respawn in 15 seconds?