Defictionalization: To promote the Dragon Bash holiday, ArenaNet produced physical versions of the dragon piņatas that appear in game and sent them to assorted gaming journalists.
Dummied Out: There was originally going to be a Canthan (Based off the Asian continent of the original game) district in the human capital of Divinity's Reach, but due to Executive Meddling (the Asian market disapproved of multiple Asian cultures being lumped together as one), the whole district was turned into The Great Collapse, which has been rebuilt into the Crown Pavilion.
I Knew It: Though the professions were revealed over a period of 18 months or so, all of them were guessed from concept art after the first reveal. The last profession to be revealed (the Mesmer) was the most popular guess for the last profession for about 10 months before it was actually shown.
The fact that everyone knew the mesmer was the eighth profession for so long without official confirmation actually caused some problems for the official wiki, with the rampant speculation filling discussion pages and hindering actions to document the information that was available.
In regards to the Queen's Jubliee Living Story event, many speculate that someone will attempt to assassinate Queen Jennah, but Plot Armor demands her to be alive. This is especially true when during the Clockwork Chaos Living Story content where Scarlet Briar, who led a group of Aetherblades attacking the Opening Ceremony, shot a rocket towards Queen Jennah during the Closing Ceremony. Luckily, Queen Jennah survived thanks to the fact that she created an illusion. Lord Faren, however, wasn't that lucky (although he does survive the fall).