Continuity is a tricky thing in Doctor Who. Due to the nature of the beast, with a plethora of writers contributing to the "canon" (a loaded, debatable term), contradictions abound. For large things, like humanity becoming aware of non-Terrestrial life, a showrunner will usually find a way to smooth over the issue with a clever (or not-so-clever) retcon. A few such instances were highlighted recently in an article at the Houston Press.
However, sometimes rather egregious inconsistencies remain unaddressed. Other times, writers throw in ideas that some fans simply find distasteful (while others, of course, couldn't care less). There is at least one of the former category—and several of the latter—that still irritate me. Here are some of the blips in the Doctor's adventures I'd like to see sorted.
I'll start with the actual discontinuity, which involves the Blinovich Limitation Effect (BLE). First mentioned in Day of the Daleks, and later in Invasion of the Dinosaurs, the BLE began as a vague hand wave to explain (without explaining) why our heroes couldn't simply go back and try again and again if they failed their mission the first time. Later, in Mawdryn Undead, we learned that an extension (or corollary, perhaps) of the Effect meant that if two versions of the same individual from different points on their personal timeline were to touch, there could be catastrophic effects.
Unfortunately, post-Hiatus Who has, more often than not, ignored the perils of the BLE—unless it happened to suit a particular storyline. There are, off the top of my head, three examples of characters crossing their own timelines and touching other versions of themselves. The first, where Rose holds her infant self in Father's Day, does actually lead to severe consequences in the form of Reapers. It's not the same result of that corollary that we saw in Mawdryn Undead, but at least we see how the paradoxical crossing of her own timestream affects Rose and her surroundings.