word-usage
I have used a sentence like "Be careful of wearing a coat." and I wondered if it was a normal sentence a native speaker would say.
Looking at the OALD, I noticed the following example:
I'm very careful about washing my hands before eating.
The dictionary adds between parentheses:
I make sure I do it.
Does that mean be careful can be used to mean "make sure"? Would my example sentence be understood to mean "Be sure to wear a coat."?
I would say "be sure to turn off the light" or "make sure you turn off the light" interchangeably, with a slight preference for the second form. I think "make sure to turn off the light" is okay grammatically. It's just less common.
This answer on ELU has some good, objective information about the relative frequency of the different phrases that's consistent with my first reaction.
The definition
obnoxious (adj.) very rude, offensive, or unpleasant : an obnoxious child; an obnoxious habit
is almost correct, but as commonly used, obnoxious also implies that the offensiveness is difficult to ignore. An obnoxious child is not just rude, but loudly or aggressively so. An obnoxious color or combination of colors is unpleasant but also intense or bright like the way the building in the linked article was painted:
I would rewrite the definition as
obnoxious (adj.) very rude, offensive, or unpleasant in a way that is difficult to ignore: an obnoxious child; an obnoxious habit
I don't know if I feel hatefulness toward an obnoxious person - I think it's too strong a word. I would describe it as strong dislike mixed with annoyance.
Best Answer
In some contexts, "to be careful" can be used to mean "I have an ongoing habit of making sure I do [x]". This is the case in your hand-washing example; "I'm very careful about washing my hands before eating" means "I always make sure to wash my hands before I eat".
It doesn't work quite the same way in a command, though. If you're telling someone to be careful about/while doing something, you're cautioning them. For example, "Be careful when you cross the street!" is said to make sure someone uses caution and isn't hit by a car, not to tell them to be sure to cross the street.
So, in your example sentence:
This doesn't mean "Be sure to wear a coat", but actually the opposite; it's a caution against wearing a coat. It would be understood as:
Which is probably not what you mean to imply!