Sometimes the first conclusion - although reasonable - is not always the corrrect one. Here's an example of just such a case, this one dealing with starting engines. Here's the scenario:
You've had five flights and get ready for a sixth. You go to crank it up and nothing happens. First thought goes to the glow plug. You remove it and check to see if it lights. It doesn't.
Conclusion: first appearances - as well as 2nd, 3rd, and 4th ones - can sometimes be deceiving.
And maybe even 5th conclusions: While it's true that over time, these things wear out, who's to say that blasting all the gunk out of the tip of the ni-starter wouldn't have gotten it working again.