I think the first one I remember was something to do with avatars. Avatars were re-enabled on April 1st after being disabled for a years,..
If it is true so theymos pranked us for 5 years every April fools day come. Every joke he makes every year it really marks those forum members who experienced it.
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Thank you guys for adding such informations to my compilation list in OP. It's now updated.
Many people didn't get it, some members created topics in meta yes meta! asking about random characters when the first topic was April Fools!
I like the way people make threads to ask about the unexpected bugs or problems they see on April Fool's day, it's really fun. And conversely, I hate people who create topics to explain about it before others have a chance to enjoy it

accidentally they lost the feeling that others might have
I tend to agree with you but if everyone who experienced or a victim of a prank will make a thread on this meta regarding to their unexpected bugs, I don't know how it looks like.

But at least there are always victims in every prank theymos has during April Fools day. And we enjoyed how forum members reacted.

I remember there is a thread that selling collectibles during April fools day using your merit to purchase. If I'm not mistaken that was the year 2017 or 2018.