The biggest catposting incident on lemmy was when people started "void posting" in cat community on lemmy.world.

Black cat images made it to the top of /all for several days. Here's the list of those images (lemmy search with cat community and with keyword "void"):

https://lemmy.world/search?q=void&type=Posts&listingType=All&communityId=29&page=1&sort=Old

They don't have as many upvotes but a year-two back lemmy didn't have as much activity as now. Still made it to /all

Edit: edited query to sort by "old" to get the oldest ones first

Edit2: sorry, got stuck looking at the voidposts and realized the biggest incident was 7 months ago, so need to scroll a bit down with that query but you should also see the voidpost memes :)