You know you overbuilt your rocket when...

[Michaelo90]

When you do a semi-powered landing test of your latest SSTO and it tips over and the explosion sends the biggest SpaceY tanks (23t dry weight) 1,5 km up in the air.

When there is nothing left on your fuel meters and you still have two orange tanks worth of fuel left.

I've done both of those with my new SSTO

Edit: When you can pull your launch clamps out of the ground. Yes, that happened too

Edited May 13, 2015 by Michaelo90

[Kendrick]

when all your craft start with 5M starlifter parts.

[yorshee]

When you have to lose a near-full tank of fuel because you're landing soon and your landing legs are attached to the next stage.

[windows_x_seven]

When make a transfer burn to Duna using the stage that was supposed to launch you from Kerbin.

[TheMoonRover]

When your mission is to rescue three kerbals from LKO, you succeed despite taking several rather sub-optimal manoeuvres to do so, then decide to fly-by the Mun. The trajectory happens to need only a small correction to get a Minmus encounter. It is during the correction burn that the launch stage runs out of fuel. The ship is capable of landing on and returning from Minmus with a single LV-N. It manages a soft landing despite only having a single parachute.

Or, when removing the monopropellant from the command pod gives you an extra 400 delta-V.

[SpacePi]

When a small "addition" to your space station costs 16M Kerbucks. Good thing for sandbox!

[M4ck]

When you use the "flea" as a separatron because separatrons don't have enough power to push spent stages out of the way.

Ha, been there, done that That's how a single launch of a 1,5kt heavy, Class-E asteroid hauler goes.

[MarkusA380]

...when you need to constantly throttle down all engines to 5%.

[waterlubber]

When you just leave the thrust limiter on .5% in the VAB because it's too delicate to change during flight

[tetryds]

When you find out your first lifter stage is actually a 60ton SSTO.

[Endersmens]

When you find out your first lifter stage is actually a 60ton SSTO.

This. I have many first stage SSTO launches. Using a skipper with jumbo 64 (well the two smaller ones put together)

[mythbusters844]

When you throttle your plane down to 2% thrust but it's still flying at Mach 1.4

Edited May 17, 2015 by mythbusters844

[waterlubber]

When you throttle your plane down to 2% thrust but it's still at Mach 1.4

Underbuilt

When you set the debug menu drag to max to stop your rocket from hitting the speed of light too early

[Prepper-Jack]

When the first stage of your asparagus launch remains unharmed on the launch pad.

[MGCJerry]

You need to create a BOINC project to help do the physics calculations for KSP because your 60,000 node farm of quad 12 core servers just isn't cutting it anymore, and you still barely break 1 frame every 30 minutes.

[hellion13]

When you hyperedit Gilly onto the top of your rocket and it still takes off and makes it to space in 30seconds or less.