Of all the Doctor's foes, none conjure up such terror among hardcore fans as the dreaded 'retcon'. But retcon isn't an alien, or a moustache-twirling villain - it's a narrative device. "Retcon" stands for "Retroactive Continuity", and it essentially describes moments that change or contradict established canon. It's such an infamous staple of Doctor Who that spinoff show Torchwood named a mind-erasing drug "retcon" in its honour.
Not all retcons are necessarily bad: sometimes they're important for opening up new storytelling opportunities or making something feasible that wouldn't be otherwise. But they can also cause confusion, nullify some of the show's best episodes and characters, and create unfortunate missed opportunities.
However, retcons are also somewhat inevitable. When dealing with a show that's spanned over 50 years, has a heavy focus on time travel, and has been headed by numerous different production teams, it would be crazy to expect completely robust continuity. Still, not having the continuity morph into something completely different every few years would be nice.
10. The New Paradigm Daleks
Early Matt Smith story Victory of the Daleks infamously introduced a brand new Dalek design in an effort to breathe new life into the iconic baddies (and sell more toys). The sleek bronze designs which had been in use since 2005 were set to be replaced with brightly-coloured behemoths, each with a different rank and function.
These were the New Dalek Paradigm, and the story set them up to be the face of the Daleks for the foreseeable future -- a point hammered home when the shiny new variants exterminate their apparently inferior predecessors and escape the Doctor's justice at the end of the episode.
Embarrassingly however, the new design was so derided by fans that it was swept under the rug by the very next series and never seen properly again. You can occasionally glimpse them in background cameos (with their paint jobs toned down) but by and large the bronze Daleks fans had grown to love were put firmly back in place as the supreme design, with no explanation for what happened to the new Paradigm. Shame, too: these Daleks never got to prove themselves, and it would've been interesting to see what devious plans Moffat had in mind for them. No second chances: we're that sort of a fandom.