(I'm a bit surprised that I haven't seen one of these, but I don't think I have...)
A collection of forum memes and in-jokes. Here's a few, just off the top of my head.
The emoji: A troll-y bus emoji indicates that the remark is deliberately troll-y and should not be taken too seriously. Occasionally other emojis with "tro" in the name are used, such as or , but the is the most common. Also used in the name of the Trolleybus Garage, our official forum category for topics likely to provoke flamewars. Content in this category is hidden from search engines and unregistered users.
and similar: Disco-horse refers to Discourse, forum software that we used to use but ended up dropping due to it being too big of a in many ways. (Alternately, has been seen, read as "disc-horse" even though the first emoji is named "minidisc.")
Shoulder aliens: Miniature entities that sit on forumers' shoulders and whisper interpretations to them that are entirely unrelated to what was actually in the post that the forumer is replying to. Forumers frequently accuse other forumers of being inspired by these improbable creatures in debate threads. The term was originally coined by @Blakeyrat.
and related: A pendant is a visual indication of a pedant, one who interprets things in a strictly literal fashion even when such an interpretation is not contextually appropriate. Depending on the situation, this can be used both as an accusation and a point of pride. A person who is insufficiently ic in a debate may be accused of listening to shoulder aliens.
And then the murders began: This sting originated in a thread by @Yamikuronue describing a purported truism that any story becomes better if the second line is "And then the murders began." Forumers picked up on it and began to insert it anywhere it might prove humorous.
Flagged for libel: Once upon a time, a certain forumer who shall remain namless caused quite a ruckus when his somewhat tenuous grasp on reality led him to flag for moderation many posts by forumers with views differing from his own, claiming that they were "libeling" him. The utter absurdity of the situation caused several forumers to pick up the phrase, and now it's not uncommon to see someone respond to a ridiculous post with something along the lines of "FLAGGED FOR LIBEL!!!"
Spellarring: A typo-rich style of spelling common in posts by @accalia, occasionally immitated by otehr forrumers for comdeic effekt.
Node: Node-baby refers to NodeBB, the current forum software, similar to the disco-horse noted above.
YMBNH: You Must Be New Here. Sarcastically used to point out when someone has said something indicating an expectation from other forumers that runs contrary to known attitudes or behavioral patterns.
TDEMSYR: That Doesn't Even Make Sense, You Retard. This one's pretty self-explanatory. Often invoked during debates.
: A standard ninja emoji. Named "hanzo" rather than "ninja" after a series of front-page TDWTF articles about an IT guy called Hans who fantasized about being the super IT ninja warrior Hanzo. (It makes just as much no-sense-at-all in context.)
Wooden table: Discussion of websites with a bad UI will often feature some mention of a picture of a document on a wooden table. This is a reference to one of the classic TDWTF front page articles, discussing the counterproductive technique of scanning a photo of a printed document on a wooden table and uploading it on your website, rather than simply formatting the contained information in HTML to make it more useful to the user.
Feel free to post more, but let's please do our best to not derail this, mmkay?