You are correct that can happen and the most common place is where then neck meets the body! The neck at the first 9 frets at the body is flat then your relief comes into it. If at any time the fingeboard flexes different then the neck a hump can show in that area. Humidity can cause it even but it does not happen often. More common on an acoustic and most times caused by the neck flexing against the fingerboard join at the body.

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Hi LesK: sounds good - but actually that wasn't the point of the steel ruler in this case.

By placing it edge on across a suspected high fret it will rock slightly like a see-saw with an audible click at either end. The status of that fret is confirmed and action can be taken. For this purpose, the shorter the ruler the better.

On the wider point, of course using the string as a straight edge as you describe is the traditional method for setting neck relief. But in fact a two foot steel ruler, again used edge on, is even better. It has no give in it as you measure the distance between the fret top and the bottom edge of the ruler as you check that gap with your feeler gauges, so more accurate measurements are possible more easily.

And it simply avoids all that business of holding the string down at the last fret with one hand whilst trying to take accurate measurements with gauges with the other hand. Same end result - but more simply achieved.

Cheers - C

Good point about the steel ruler, thanks for posting.