I've been compiling a list of maltreatments of the English language. Here are my top 10 so far.
compatability; definate(ly); desparate(ly); seperate(ly)
You mean compatibility, definite(ly), desperate(ly), and separate(ly) (respectively). The first one arises from confusion with "ability", and certainly the conflation of "desperate" with "separate" is easy enough. I don't know what excuse there is for "definate".
A compliment is praise. A complement is the inverse of something (for example, 0b101010 is the
two'sone's (thanks, CHz!) complement of 0b010101, and a film negative is the complement of the positive print).
When you affect something, you effect a change in it.
Can't happen. A person is accountable if they can be held to account.
Classified what? USDA Choice? The correct usage is "classified secret".
X Department (of the United States Cabinet, e.g. State Department)
You wouldn't say "United States State Department", would you? No, you wouldn't, because that sounds redundant. It's "Department of State". Also "Department of Defense", "Department of Energy", "Department of Justice", etc. Some of the Departments have a "the", like the Department of the Interior. But deviation from the rule doesn't go beyond that.
Dilemma (as problem)
A lemma is a course of action or line of reasoning (dictionaries define it as a subsidiary proposition … used to demonstrate a principal proposition; in this case, the principal proposition is often a course of action). A dilemma is two of these, from which a person must choose. Not every difficult problem is a dilemma.
"Welcome to …" (telephone system, website, etc.)
"Welcome to" should only be used for a place. "Welcome to Huntington Beach, population 189,594". It should never be used for anything else, including web sites and phone systems. When I hear "Welcome to (mumble). Please choose from the following selections:", I want to reach through the phone and unplug that answering system.
Lower 48 states, Continental United States
First, Hawaii is lower than the lower 48. Technically, the lowest 48 states include Hawaii and exclude Maine. Also, "lower" would be only correct when there are two (e.g. "lower case" vs. "upper case"). We have never had fewer than 13.
As for "continental United States", you have to include Alaska on this one. It's on the same continent.
Just use "contiguous United States".
x, y and z
No, no, and no. Use the "serial comma", the comma after the second-to-last element in the list. It just looks better that way. (This does not apply when the list has two or fewer elements.)