I just noticed that I appear to have received two 'necromancer' badges before I even gave the corresponding answers: for instance, compare the dates on the list of badges with the date on this answer. Upon further inspection, it seems that the system just mixed up different answers which provided me with a 'necromancer' badge. For example, the coincidence of the dates makes it plausible that the answer I linked above and this other answer got switched or confused in the 'necromancer' badge list.
Afterwards, I looked for examples involving other users and other badges. As an example, I tried Tom Goodwillie (just because Tom has lots of badges) and the results were fairly confusing: take a look at the lists of 'necromancer' badges, 'enlightened' badges, and 'nice answer' badges. For a badge relating to questions, try this long list of 'nice question' badges. I cannot immediately find a precise pattern, but a lot of these badge lists appear to approximately reverse the real dates.
Edit: I wonder if this issue is related to this answer on meta.stackoverflow. That answer implies that until some time in 2010, the information about which answer or question awarded a given badge was simply not recorded.
