Stupid things characters do to the sake of the plot

[Robert Jones]

No: HPFGUIDX 94534

I finished rereading COS last night, and I noticed that some of the 
characters do stupid things for which there is no explanation except 
that they are necessary to advance the plot.  Here are two examples 
from COS.

At the train station, when the wall at Platform 9 and 3/4 wouldn't 
open, Harry and Ron almost immediately run to the car.  I know they 
had to get the car into the Forbidden Forest at Hogwarts for later 
in the story, but shouldn't they just have waited for Arthur and 
Molly?  (And by the way, where were Arthur and Molly?  Didn't they 
notice that Harry and Ron didn't get through?  And shouldn't there 
have been other wizarding parents dribbling through the wall a 
couple at a time?)

At the end, Lockhart blabs all about the fake he is to Harry and Ron 
and then immediately turns around to put a Memory Charm on them.  I 
know he had to do so that Harry and Ron could get him down in the 
Chamber of Secrets to finish off the Lockhart story line and to get 
the ceiling to crash, but still that is just stupid for him to do.  
He had kept that secret for years and now was leaving and yet he 
still blabs away to a couple kids and then would have to explain why 
Harry and Ron lost their memories in his office.  (And by the way, I 
know it was hard to get a DADA teacher, but shouldn't the great DD 
have known Lockhart was a fake?)

Any other examples?