Name's the Same: Tolkien changed names of characters and locations at least once with every draft or linguistic revision, and also recycled old ones. Names that would later be applied to well-known characters in The Lord of the Rings show up many years before that book was conceived, attached to completely unrelated characters.
For example, one Gimli first appears in the "Tale of Tinuviel" (written in 1918-1920) as a "Gnome"note which, strangely enough, at this stage was a synonym for Noldorin Elf and fellow prisoner of Beren in Tevildo's kitchens.
Legolas Greenleaf was first the name of an elf leading the fleeing people of Gondolin over the plains of Tumladen and over the pass of the Cirith Thoronarth.
Even within the Silmarillion part of the legendarium, names sometimes switch places bizarrely. For a short time Beren was named Maglor.
Science Marches On: Tolkien firmly believed that space travel was physically impossible; in The Notion Club Papers, which takes place in 1987, it still hasn't been accomplished. About a decade later, Sputnik would prove him wrong.
Write What You Know: "The Fall of Gondolin" from The Book of Lost Tales part 2 is clearly inspired by Tolkien's then-recent experiences in World War One, down to Morgoth's mechanical dragons evoking the early tanks used in the war.
Write Who You Know: "The Notion Club Papers" is an Affectionate Parody of Tolkien's own experiences with the Inklings—in particular, Michael Ramer resembles C. S. Lewis and Alwin Arundel Lowdham resembles Tolkien himself. Also, Author Avatars Alwin Arundel Lowdham from that story and Alboin Erroll from The Lost Road are both linguists.