Be careful
1. an instruction to take care in a particular situation. Bill: I'm going to the beach tomorrow. Sally: Be careful. Use lots of sunscreen! Jane: Well, we're off to the Amazon. Mary: Heavens! Be careful!
2. a way of saying good-bye while cautioning someone to take care. John: See you around, Fred. Fred: Be careful. Alice: Well, I'm off. John: Bye, Alice, be careful.
careful not to do something
using care to avoid some difficulty or difficulties. Please be careful not to discuss politics with Mr. Brown. Please be careful not to leave the house unlocked.
careful (with something)
treating or handling something with caution. Please be careful with the vase. It's old and valuable. That's too big a load. Be careful!
If you can't be good, be careful.
Prov. If you are going to do immoral things, make sure they are not dangerous.; If you are going to do something immoral, make sure to keep it secret. (Sometimes used as a flippant way of saying good-bye.) Be a good girl on your vacation trip. Or if you can't be good, be careful. Ernest likes to close his letters with, "If you can't be good, be careful."