Answer:
The scenario "Bob didn't wear his lucky T-shirt to class, so he failed his chemistry exam" illustrates the fallacy of "post hoc reasoning."
Post hoc reasoning, also known as "post hoc, ergo propter hoc" (after this, therefore because of this), is a logical fallacy where one assumes that because one event followed another, the first event caused the second event. In this case, the assumption that not wearing the lucky T-shirt caused Bob to fail his chemistry exam is an example of post hoc reasoning, as there may be many other factors contributing to his exam performance.
:)