These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Broken Base: The late 1990s saw a drama-filled political schism among developers and players alike, leading to a splinter MUD called Lost Dawning (originally a "tutorial MUD" for Lost Souls) that was dedicated to what Lost Souls had been before it was Ruined Forever by a developer called Chaos. (Chaos now runs Lost Souls completely, and Lost Dawning is dead.) Another base-breaking came circa 2004 when massive code revisions remade the game so as to be nearly unrecognizable to older players.
Demonic Spiders: Probably best exemplified by scrub NPCs with "amulets of retribution", items that hit you back (once) if you hit their wearer with an attack of a specific type. Also literally, as with the rhax species, and depending on how broadly you want to apply "demonic", the spider lloigor Nachthla.
Epileptic Trees: The complexity of the systemry regularly leads players to come down with fits of contagious apophenia. A historically popular topic for this has been the effects of different colors of flux weapons.
Goddamned Bats: Rats and bats can be a bit of a pain for newbies because of their dodginess.
Serial Numbers Filed Off: A trend for the past few years has been to rename things that were originally based on other people's intellectual property in some way.
That One Boss: The spider lloigor Nachthla, a being of primal power whose idea of fun is making physical forms for itself that embody the principle of predation.