[Lawtan]

Lost
In the early days of Doctor Who, the companies would purge their material as a standard money-saving practice. (Especially true of Black and White material - this was the case until personal videos could be sold). Many we only have because exported tapes (for Asia, Africa, Germany, South America) didn't destroy them, and later searching found them.

Sort of sad, because the Second Doctor was affected the worst - from the first regeneration into the Second Doctor to the second-to-last episode.
And 2 is one of my favorites.