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Consider the following quote from a book I've been reading:
…[W]e're…given the…information that when he's poorer, he eats fewer kidneys. (Actually we're told that when he's richer, he eats more kidneys, but that comes down to the same thing.)1
Does it actually come down to the same thing? No, you might just as well say that when he's growing, he's getting older comes down to the same thing as when he's not growing, he's getting younger. To return to the example, it might be that he likes kidneys so much that losing money has no effect on his kidney-eating―maybe he cuts back on liver so that he can afford them. Alternatively, when he gets poorer he might even increase his kidney consumption to console himself over his loss of wealth. We need more context to determine what effect decreasing wealth might have on his diet.
This is an example of an improper transposition2, that is, treating a statement of the form When A then B as logically equivalent to the statement When not A then not B.
Now, if his getting richer varies directly with the amount of kidneys he eats3, then what the quote says would be true. I suspect that this may have been what the author was thinking when he wrote the above passage, but what it says is logically weaker.
By the way, despite the above error, the book has been very interesting so far.
Notes: