Assuming you mean the one labeled "voltage divider", the problem is that it doesn't actually divide voltage because the input and output are switched. The unloaded output voltage is the same as the input voltage.
Sure thing! One of the things that helps me sometimes is re-arranging things visually.
A real divider:
Input o----vvvvv----o-------------o Output
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Z
Z
Z
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o
Ground
The bad one:
Input o-------------o----vvvvv----o Output
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Z
Z
Z
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o
Ground
Looking at it this way, it might be a bit easier to see that in the good circuit, both resistors can be in a Kirchoff voltage loop that includes the input, while in the bad one they can't.
(Edit: it appears I don't understand linebreaks on HN, so things are gonna appear garbled until I fix it.)
