Hugh: Hey, did you go shopping today?
Marty: Yeah, I went to Costco.
Hugh: Did you remember to get toilet paper?
Marty: Yeah, I got four packages.
Hugh: Cool. Where are they?
Marty: In my car. I need help carrying them up.
Marty: There's 30 rolls in each pack.
Hugh: Whoa! You didn't need to go off the deep end!
Marty: You said to get a few packages.
Hugh: Yeah - the ones with like 6 rolls! Now we have enough toilet paper for years!
Meaning: The idiom "to go off the deep end" is used when someone does something beyond what is expected, usually crazy or irrational. In the example above, Marty went off the deep end when he bought 4 large packages of toilet paper at Costco. The idiom is also used when someone becomes suddenly angry or upset, as in the next example:
Chrissie: How did the presentation go with the bosses?
Pete: Not great. They didn't really like our idea.
Chrissie: They didn't like any of it?
Pete: Not really.
Chrissie: I've been working on that for a month! Do you think they're mad? I hope I don't get fired!
Pete: Whoa! Don't go off the deep end. They appreciated the work we put into it, but they had some other ideas, and they want us to work together on a new presentation.
Chrissie: Oh good. Sorry, didn't mean to panic there.
Meaning: In this example, Chrissie panics because her bosses didn't like the project she and Pete had been working on for a month. However, her sudden panic is irrational, and Pete calms her down by telling her that they want her to work with him on the new idea.