Trying to word things as un-spoilery as possible but if you're touchy about spoilers for the BBC Sherlock, SPOILER WARNING.
Driving myself off the deep end reading (WAY impossibly complex) theories on the Sherlock cliffhanger/finale, that I would reject entirely as just wishful thinking and overanalysis if it weren't then for the question
then why did they put it onscreen? If it's not arguably production error or whatever, and obviously a conscious production choice, something that SOMEONE had to PUT in the show...
http://finalproblem.tumblr.com/theory-index has a GREAT compilation of theories, both grandiose and minor, on The Reichenbach Fall and Sherlock more broadly. Moffat and Gattiss both are such high-calibre writers, and have hinted at such complex and subtle clues, that I believe, or at least hope, that at least some of the theories are true.