C'mon, what's the dumbest thing you've done which seemed a good idea at the time?
The first one that springs to mind - and there will be many more I'm sure - was to think it would be a good idea to chip a golf ball over our conservatory roof. Fortunately the shot was bad enough to hit the huge glass pane (roughly 6 feet by 4 feet) right in the middle so it didn't break. Nearer the frame and the vibration would've caused it to shatter and cost a fortune to replace.
After I saw the original Superman film I thought it would be a good idea to fly on to my bed like Superman.
I didn't realise that my brother had left a sheet of glass on my bed.
Ooops.
To his credit he did give me a piggy back to A & E.
I was dared by friends to use a pogo stick on an icy driveway...you can imagine how that turned out...
I joined some website - "British Sitcom Guide" I think it was called. Fortunately it doesn't exist any more.
I dared my sister to jump out of a first floor window when we were kids... she nearly killed herself. I then dared her to jump into sinking mud... she nearly drowned. 
She saved my life when we were scuba diving a couple of years ago... so I owe her BIG TIME.
When I was a less old I bought my first padlock - I used it as a key ring. The only key I had was the one for the padlock. Good idea, I thought. This way I can't lose it. I soon realised I couldn't use it, either! So I picked the lock, which was easy, freed the key and decided that a padlock that was so easily picked wasn't worth having. I chucked it away. (This is a a very nice thread, Tuumble)
Had my hair permed.
No you didn't, did you?
I know it's hard to believe but yes, I did, Ellie.
Why?
Do you need a padlock?
I was persuaded by a smooth-talking LA hairdresser to get my hair straightened. Came out looking like a horse mane and smelled like a chemical toilet for weeks afterward.
Quote: Stephen Birch @ May 13 2010, 10:06 AM BSTWhen I was a less old I bought my first padlock - I used it as a key ring. The only key I had was the one for the padlock. Good idea, I thought. This way I can't lose it. I soon realised I couldn't use it, either! So I picked the lock, which was easy, freed the key and decided that a padlock that was so easily picked wasn't worth having. I chucked it away. (This is a a very nice thread, Tuumble)
You really are as barmy as you look aren't you. 
The time I snapped a cd in half because I was annoyed that the recording had failed.
You only do that once, believe me.
Quote: Stephen Birch @ May 13 2010, 10:18 AM BSTI know it's hard to believe but yes, I did, Ellie.
Why?
Do you need a padlock?
Think you may have missed Chip's post in between.
But actually yes, I would like a padlock. Off on hols soon.
Quote: Oldrocker @ May 13 2010, 10:51 AM BSTThe time I snapped a cd in half because I was annoyed that the recording had failed.
You only do that once, believe me.
What happened? Did you lacerate your wrist and wind up on deathwatch?
Quote: Suzy @ May 13 2010, 9:20 AM BSTSubstituting a generous squeeze of Fairy Liquid for a dishwasher tablet when I ran out.
Did it work...?