For April fools in 2011 locked chests were added that opened a link to a fake Minecraft Store webpage with fake items for sale. The earliest archived versions of the webpage already have a big "april fools" banner that was added the following day and most of the original page elements don't actually show up:
what the archive looks like: https://web.archive.org/web/20110402233948/http://www.minecraft.net:80/store/loot.jsp
what the original looked like: https://minecraft.wiki/w/File:Minecraft_store_april_fool_1.png