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You would think that correctly checking the oil dip stick on an engine wouldn't be rocket science; there's only one right way to do it, right?
It turns out that's not the case. I went flying this last Saturday, and it turns out that I checked the oil wrong during my pre-flight, before starting the engine. The reason I discovered this was because after taxiing to the gas pumps and filling up for the flight, I checked it again (just to see what the oil looked like when it was warmer), and it was quite a bit lower.
The first check was with the engine completely cold. The temperature at the time of my pre-flight was between 40 and 50 degrees F. The oil was dark colored and pretty thick; when I checked the dipstick, the oil covered the 6 quart mark. (I was flying a Cessna 150: 4 qt minimum, 6 qt is full).
I started the engine, got it running on the second start, and taxiied to the gas pump, and filled up for the flight. Just out of curiosity, I checked the oil again (I'd probably run the engine for 5 minutes at this point). It was much runnier and lighter colored. And there was only 4 and a half quarts of it. Plenty to fly on safely, but not what I'd thought. Someone else in the club had mentioned that plane had been using oil, so I added a quart. After some flying (maybe 45 minutes), I checked the oil level again. 5 1/2 quarts this time, basically none lost since I added some.
As I was buttoning the plane up, the next flyer came by and started to prepare for his student flight. I mentioned the dipstick giving two different readings to him, and he said "yes, when the engine is cold, you always have to wipe the dip stick to make sure the reading is accurate.". And that's our engineering moral for today.