Martha Stewart believes you should clean your dishwasher. If you have dogs, you also know that even that spendy Dyson Animal needs a good cleaning once in a while because, frankly, dogs are damn gross. This is mainly because even though we assume our cleaning tools are by nature clean, it's a wrong assumption.
We recently moved into a new apartment which has wall-to-wall carpeting. The previous residents used Carpet Fresh. How do I know? Because today I went to empty the bin on our new vacuum cleaner it was full of carpet fresh like stuff. But we've been sucking it up since we moved in, it hangs around in your carpet forever and starts to stink after a while. That there was funk in the bin didn't surprise me. I also noticed that there was some dog fuzz in the part of the bin that caused the dirt to spin around. I popped it out to unwrap the hair and *plonk* out came a giant mess of dog fuzz bound with a huge amount of Carpet Fresh. Dust flew everywhere causing me to get insanely pissed. It also made me think "Huh, that's why every time we vacuum it smells worse.. I just assumed we were kicking up leftover Carpet Fresh from the carpet." I took everything apart, cleaned it, and put it back together. Now things work better. I knew that this was something I really should do once a week, but I got lazy and assumed that my shiny new cleaning tool was by nature, clean.
So how does this relate to system administration? We assume a lot in our line of work. We assume (or perhaps "believe") that things break in the same way, that data formats are correct, that things that do have inter-relations don't, that someone else hasn't changed that script in the past year. That cleaning tools don't also need a good cleaning once in a while. When we don't check what we definitively don't know about a system or process before we build upon those assumptions, we'll get bitten at some point.