Katakana is a Japanese script used for foreign loanwords to describe stuff that is not native in Japanese. There’s a host of katakana words that are pretty much similar in meaning to their foreign equivalent - if you can grasp their pronunciation (which can be quite a challenge), you should have no problems understanding them. Unfortunately, there’s another group that have their original meanings corrupted to a certain degree. This article lists some of them.

マンション mansion – No, it’s not the huge, gorgeous house with a garden or two that you’re envisioning. Basically, a “mansion” in Japan refers to a slightly bigger version of an apartment.
ハイテンション high tension – Refers to “a highly excited state of mind”. It has nothing to do with high tension wires, so don’t get shocked if you hear a Japanese say “この子たちはハイテンションですね～ (These children are really high tension!)”.
ワンパターン one pattern – Refers to a repetitive motion or action. For example, a tiger that keeps walking around a cage for five minutes using the exact same route can be called “one pattern”.
キーホルダー key holder – keychain.
イメージ image – imagination.
ゲーム game – Its usage is restricted to only video games, computer games, console games, mobile phone games – you get the drift.
ハンバーグ Hamburg – short for “hamburger”.
フライドポテト fried potato – French fries.
サンド sand – Short for sandwich. Sure sounds appetizing, huh?
アイス ice – Short for “ice cream”. Where did the “cream” go to?
ソフトクリーム soft cream – soft ice cream. They omitted the “ice” this time. Why can't they, for continuity's sake, name it “soft ice”?
オーバー over – It refers to an “exaggerated action”.
プラスアルファ plus alpha – Means “additional”, “extra”.
テスト test – Its usage is restricted to school paper tests.
ドンマイ dun my – A contraction of “don’t mind”, which means “it’s okay, don’t mind it”. It’s usually used as a form of encouragement after someone experiences a setback of sorts.
ファイト fight – Its meaning is more or less the same when used in video games, but in the real world, it’s used as a form of motivation or encouragement. Basically, it indicates “keep going”, “don’t give up” or “hang in there” when someone is faced with a rough situation.
ゴールイン goal in – No, it has nothing to do with soccer or a sport of any kind. It means “get married”. Now where did it come from?
マフラー muffler – Used only to mean “scarf”. The Japanese don’t use the word “scarf”, by the way.
パンツ pants – Refers to “panties”. Really. No kidding.
ハート heart – Refers only to its figurative meaning. “Shinzo” is used to refer to that physical object that’s beating inside your body.
カンニング cunning – Means “cheating”, and its usage is restricted to tests or exams.
アバウト about – Refers to either someone who's sloppy, or something that is not really accurate.
ジャスト just – Only has the meaning of “exactly”. E.g. ５時ジャスト “just five o'clock” means “exactly five o'clock”. ジャストフィット (just fit) means “perfect fit”.
ギブ give – A shortened version of “give up”.
ハーフ half – Refers to a person who’s half-Japanese.
セーフ save – Only used to mean “a no longer dangerous situation”, but the usage is restricted to game contexts.
タレント talent – Refers to a TV personality, who may not necessarily be talented in the real sense of the word.

What makes it worse (or funnier) is that you can actually see some of the words written and spelt as they are in “Engrish” – or “Japlish”, if you prefer. For example, there could be a huge banner outside a steak restaurant with the words “Special Hamburg Set” on it. Or, you might see “strawberry sand” in a convenience store. So if you happen to be in Japan, and see or hear some of the above katakana words, you know exactly what they mean now…