TAGAP (The Apocalyptic Game About Penguins) is a side-scrolling freeware Action Game in the style of Abuse made by Penguin DT, an indie game dev studio. TAGAP 2 was recently released as well.The website is here.
Mirror Boss: Pedro's final fight qualifies. It uses most of your weapons, either your current one or a chaingun, and uses One-Ups. Lots of them.. You have to break the rules and use the Disintegrator, which makes One-Ups useless.
Mook Maker: One of the minibosses, and one of the bosses. In the latter case, they spawn fast enough that it's a Timed Mission to kill the boss before you drown in penguins.
More Dakka: the quad damage powerup. In the sequel, iit is possible to combine it with another power up that enhances your ratio of fire and gives you infinite ammo.
The most blatant one is to the Xbox and the Red Ring Of Death. There is even an achievement.
Situational Sword: the EMP rifle. Completely useless on organic enemies and Mecha-bosses, and the secondary fire is basically useless in combat. It's also required to grab some hidden items, stops most mechanical enemies, destroys shields, and if you are a really good shot (or just are in Overdose), you can wreck Mechapenguinators.