Affirming the consequent

What it is

Reasons "If A then B; B; therefore A". Other causes could have produced B.

Example "If it rained, the grass is wet. The grass is wet, so it rained." (It could be the sprinkler.)

Confirmation bias bias
Seeing a result that fits our theory and concluding the theory is true is affirming the consequent, which is how confirmation bias feels like logic.
Non sequitur fallacy
Affirming the consequent is a classic formal non sequitur: the conclusion looks like it follows but does not.
Denying the antecedent fallacy
They are twins: both are invalid ways of reasoning from "if A then B".
Wason selection task bias
The Wason selection task shows how often people test "if P then Q" by looking for confirming cases, which is the pattern behind this fallacy.
Affirming a disjunct fallacy
Both are formal fallacies where the conclusion seems to follow but does not.