You know you overbuilt your rocket when...

[MGUARN]

Your First stages become emergency fuel stations as they are half full.

[worir4]

When you have to use the Air Traffic control tower as a strut...

[Red Iron Crown]

When the pad is destroyed when testing the uppermost stage by itself.

[gameguy600]

When your last stage has the potential to keep firing for 13 years at full throttle.

[worir4]

When your craft can lift the weight of Red Iron Crown's rep!

[kaboom1212]

...when your craft weighs so much it clips through the ground and explodes...

Yes. It's happened..... Don't ask.

[lextacy]

when your Mars rover is just 15,000 meters from landing and you havn't busted open your fairings yet LMAO! And its upper stage has 70 % of its fuel left when its time to skyscrane.

[worir4]

When you attach the SPH as the cargo bay for your space plane....

[Johno]

"Excuse me, sir? An irate Russian man called Tsiolkovsky has just called. He wants to give you a piece of his mind. Shall I put him through?"

[Hary R]

when you ssto you way to layth...with a mun mission rocket.

[Madrias]

When your Launch Escape System has enough delta-V to get you to Eeloo and back, carrying all of your science equipment.

Also, when you can officially claim colonization of a world with your exploration ship, carrying a five-digit sum of Kerbals onboard.

[cantab]

When you ask Whackjob for the Arkingthaad .craft file - because it would make the perfect sepratron for your boosters.

[EdFred]

...you can cook dinner on your PC due to the overheating of your CPU during launch.

[cantab]

You cool your CPU with hydrogen and it doubles as a NERVA engine.

[lextacy]

when you use previously built rockets as sub-assemblies to use as boosters for future launch vehicles

[LABHOUSE]

When it does this Notice how the stability enhancers are off the ground and still connected.

And another:

When it can go to 10 metres on jool and get back to kerbin in one stage (when you started at kerbin and lost 0 stages).

Edited December 4, 2014 by LABHOUSE

[worir4]

Lol

When your rocket has Kerbin as it's payload.

[ThatGenericUser]

...your Mün lander is able to land on Duna and come back.

[Hary R]

When it does this http://i.imgur.com/xA8E5xA.pngNotice how the stability enhancers are off the ground and still connected.

And another:

When it can go to 10 metres on jool and get back to kerbin in one stage (when you started at kerbin and lost 0 stages).

How did you do that?

Oh I see... when you can test stability enhancer landed anywhere... mun, duna, tylo... anywhere.

Edited December 5, 2014 by Hary R

[Sli 41]

Your space station ends up with an unplanned gigantic tank full of fuel attached to it.

[worir4]

When your rocket is so tall all you have to do is decouple the uper stage and there you are, on the moon!

[Red Iron Crown]

When your rocket is so tall all you have to do is decouple the uper stage and there you are, on the moon!

Funny you say that. In the dark early days of KSP, before Hyperedit, there was a mod that added a ridiculously sized part that, when decoupled, put you instantly into LKO or MTO.

[Guest]

When you decide to fly your Mun lander to a short Minmus trip after landing the Mun.

[Fr8monkey]

When you realize you really didn't launch your rocket... It is staying still and your exhaust is pushing Kerbin away.

[cantab]

Those dots in the background aren't stars. Those are giant Kerbodyne tanks, KS-25x4s, and KR-2Ls.