Hedgewars has a couple of easter eggs (hidden features which are not really meant serious), and I will report them here.

All info in this post are according to Hedgewars version 0.9.21.

Special dates
When Hedgewars is started on a certain day (according to system time), some visuals may be changed.

• April 1: All hogs will look like tomatos (Hat “fr_tomato”) and a tomato juice effect is shown when they are hurt.
• April 20: All hogs which normally have no hat will wear the “eastertop” hat (broken easter egg).
• October 31: Main menu is Halloween-themed and hogs which normally have no hat will wear the fr_pumpkin hat.
• December 24 - December 26: Main menu is Christmas-themed and hogs which have normally no hat will wear the Santa hat.

Background info on the dates
The April 1 easter egg refers to an April Fools joke from 2014.
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31th October is not only Halloween, but also Hedgewars' “birthday”. Hedgewars has been started on 31th October 2004.

Unicorn attack
If you hold down the [Precise] and [Switch hog] keys while using kamikaze, the hedgehog will turn into an awesome rainbow unicorn.

Hidden achievements
A couple of so-called achievements are awarded for several special tasks in missions:

• Prestigious Pilot: Finish the RC Plane Challenge with only 1 RC plane (very difficult but possible)
• Naughty Ninja: Kill Instructor in Dangerous Ducklings (demo)
• Energetic Engineer: Finish Bamboo Thicket in less than 6 turns without using any bazookas (demo)
• Lively Lifeguard: Rescue all 8 hedgehogs in That Sinking Feeling *
• Rope Master: Finish Basic Rope Training in under 50 seconds

* = This achievement does not work when you use a language other than English; this is a bug in 0.9.21. See here for a bug report including a proposed bugfix: https://code.google.com/p/hedgewars/issues/detail?id=920

Note that Hedgewars does not really have a proper achivements system yet. The achievements are only recognized in the ticker or in the stats screen.