StarCraft II’s Marine unit has a voiceline triggered by clicking the unit repeatedly: a reference to the classic StarCraft I marine response lines, which themselves became internet memes. The line is directly from the original game, recorded in the same style, serving as a tribute to players who had spent time in 1998 clicking marines until they complained.
This was standard Blizzard easter egg procedure. What was less standard: a Zergling in the campaign mission ‘The Devil’s Playground’ — which takes place on a lava planet with rising flood cycles — will, if given a move order to a specific lava-adjacent tile after the first lava surge and before the second, produce a unique animation not found anywhere else in the game: it dances.
The dancing Zergling animation was discovered by dataminers before release and was initially believed to be a developer test animation that leaked into the build. Blizzard confirmed post-launch that it was intentional — placed specifically for the one player per ten thousand who would order a Zergling to dance on a lava planet mid-mission.

Really fascinating breakdown — I had no idea this was hidden in plain sight the whole time. Going back for another playthrough immediately.
The amount of craft that went into hiding this detail is remarkable. Pure game design artistry.