Assuming you believe everything the Brits have to say on the matter, check out this article: http://www.dailymail.co.uk/news/article-508025/The-babies-born-just-minute-apart-ARENT-twins.html
According to the article, the women had a "rare case of superfetation, where the body carries on releasing more eggs for fertilisation after a pregnancy has already started. It meant that, technically, [she] could have become pregnant again each month during her pregnancy."
More importantly, it assumes you're doing the deed without protection or the cover is broken -- the former of course is never advisable and the latter is never desirable...