A Saturday Morning Breakfast Cereal linked to by David Craig and Dan Everett on Facebook:
This could have been framed as a valid inference — but it isn’t here.
A valid inference:
If P is false, I will be sad.
I will not be sad.
Therefore, P is true.
Modus tollens gives us not(P is false), which is equivalent to P is true.
But in the SMBC version, the second premise is modalized, to an expression of a wish or desire, rather than a simple assertion. The most the speaker could conclude is that he wishes that P is true. So what we’re seeing is the triumph of wishful thinking.