You've been dumb before, but this time you really did it.
7 seconds sound clip from the A Charlie Brown Christmas movie soundboard.
You can hear this line at 00:19:48 in the Blu-ray version of the movie.
Quote context
[...]
- Boy, are you stupid, Charlie Brown.
- What kind of a tree is that?
- You were supposed to get a good tree. Can't you even tell a good tree from a poor tree?
- I told you he'd goof it up. He's not the kind you can depend on to do anything right.
- You're hopeless, Charlie Brown.
- Rats.
- You've been dumb before, but this time you really did it.
- I guess you were right. I shouldn't have picked this little tree.
- Everything I do turns into a disaster.
[...]