Underwater vs. Under Water
It is usually one word, but some still make it two words when it follows what it modifies.
Heighth
It’s considered incorrect, but it’s not the abomination many people seem to think it is.
Caveat
= originally a warning, now a qualification.
Droll
Droll originally meant amusingly odd or whimsically comical. It’s now widely used to mean deadpan, sarcastic, muted, or dull.
Radical
favoring revolutionary changes.
Enormity vs. enormousness
Enormity = (1) wickedness; (2) enormousness. Enormousness = the quality of being great in size, number, or degree.
Grinded
The verb ground is usually uninflected, but grinded has gained ground in some uses, especially where the word means to overcome adversity by playing hard.
Unique
It is traditionally an uncomparable adjective, but many English speakers now use it to mean remarkable or unusual, making it comparable.
One-time vs. onetime
In North America, one-time means occurring only once and onetime means former. This distinction doesn’t exist outside North America.