What's your most common KSP mistake?

[Minmus Taster]

Mine is antenna forgetfulness.

[Jetpackmaniac]

I also struggle with antenna, either I forget them all together, or I forget to deploy them in LKO and send my probe off to the far reaches of the solar system. Then I realize my mistake as soon as I’m done time warping and notice my lack of connection.

Edited June 23, 2021 by a_space_oddity
Better wording

[KerikBalm]

forgetting a docking port, or a probe core entirely sometimes...

[king of nowhere]

i really can't pick one. i'd say "all of the above".

actually, working with kerbalism, which includes a science rework, i may also add "forgetting to change the science experiments and life support on crew modules"

[MrLivion]

I always forget reaction wheels/RCS and also struts

[Wizard Kerbal]

Forgetting RCS fuel, MysteryGoo and Ladders.

[James Kerman]

I have forgotten to add the tourists to the vessel many times.

[djr5899]

• Running out of fuel on manned rockets with no docking port to transfer fuel.  (EVA Construction makes that a bit easier now)
• Forgetting to stow antenna or solar panels, and having them break due to heat/aero forces.
• Forgetting to strut boosters.

[adsii1970]

I have a checklist I use to help me remember things such as:

• Parachutes (heh, yeah)
• Batteries
• Solar panels
• Antenna of some sort
• Monopropellant

Yeah, it's a mental checklist. But those early days in .22, I would launch my main mission to realize I needed to immediately launch a rescue mission. Never leave a Kerbalnaut stranded...

[Gamera Obscura]

Batteries. I very recently had to abandon a mission before I even left Kerbin's lower atmosphere because the probe died while I was still entirely under the power of SRBs.

Edited June 23, 2021 by Gamera Obscura

[Rylant]

This is something I do quite regularly.  I try to set up a rendezvous with a station, or satellite or something with the intent of docking, or repairing it, or rescuing somebody.  Something.  I always line up our orbits and try to make my orbit 20 or 30 km lower or higher than the target, to set up an easy rendezvous.  At this point, I often realize that our orbits are 180 degrees off essentially reversed;  he is going around the celestial body one way, and I am going around the other way.  If I had realized 20 Kerbal days ago, I could have set up a slight correction burn to move my orbit to the other side, but no...  I know, the orbit is dimmer in the direction you are heading... I have done this countless times, you would think I would learn.  It's embarrassing...

 

Rylant

[Cavscout74]

While I have made all of those at one time or another, staging errors remain the most common.  Usually something relatively minor - sepratrons lighting on the wrong stage or a LES popping off early - but I've made some colossal staging errors as well (2nd stage engine accidentally added to an earlier stage).  The worst ever was killing a load of 8 tourists on the pad due to staging error - that actually prompted me to not only install but fully test launch escape systems on my manned craft, so at least the tourists didn't die in vain.

My first big error was lack of parachute on the very first probe I ever sent to Eve.  And since it only had an 88-88 folding antenna, I couldn't even send back atmospheric data on the way down so it was a total waste.

[Caradhtinu]

Usually staging, but today I found out Jeb snuck in a probe-controlled rescue ship. Made the rendezvous, got the rescuee to the hatch of the pod and couldn't get in because it was already occupied. As a punishment, made Jeb deorbit inside the derelict and jump off once the flames were out. Good thing Kerbin is so smol

[eddiew]

Staging. Always staging. I don't get it the right order until the first stage is burning and the next spacebar will destroy the vessel.

[LHACK4142]

Forgetting to put power generators on probes.

[Geonovast]

I almost never use RCS for pitch/yaw/roll.

Know what else I almost never do?  Remember to disable pitch/yaw/roll in the actuation toggles in the editor where the rules of symmetry still apply.

 

I've even reverted a launch after getting to orbit to fix the craft file, then completely forgotten what I was doing in the 4-5 seconds it takes the VAB to load and then just launch again without adjusting... many, many times.  I've yet to find a way to get those settings off by default.

[Ben J. Kerman]

I am not exactly sure what my most common mistake is, but a common is building a plane/SSTO, going on a test flight, and halfway down the runway I realize: I have no control surfaces!

Oh, and staging.

Edited June 24, 2021 by Ben J. Kerman

[SBKerman]

Controlling from the wrong probe core. I've lost numerous vehicles by having focus on a core that's pointing in the wrong direction.

I also have a tendency to forget mission objectives. I'll launch, carry out the mission, then land and recover only to realise that I didn't do half the things I'd planned to do.

[KerikBalm]

19 hours ago, Gamera Obscura said:

Batteries. I very recently had to abandon a mission before I even left Kerbin's lower atmosphere because the probe died while I was still entirely under the power of SRBs.

I rarely forget to bring enough power, or a power generator... but the other day while iteratively testing and developing a reusable 2 stage rocket for a 3x rescale, I ran out of power for the 1st stage shortly after decoupling the 2nd stage... yet I hadn't forgotten a power generator.

The first stage recoverable booster was massive, it had some vernors, but also at least 4x2.5m reaction wheels. I had given it a single RTG to supply power. I hadn't given it a single battery, so all it had was something like 15 units of EC from the probe core. I told the SAS to turn retrograde for the burn to send the trajectory back to KSC, before engaging RCS... and promptly lost control of my rocket as the reaction wheels sucked up all the EC much faster than the RTG could supply it.

I missed my burn at apoapsis, and the next iteration had a 2.5 meter battery...

[Jetpackmaniac]

8 hours ago, Geonovast said:

I've even reverted a launch after getting to orbit to fix the craft file, then completely forgotten what I was doing in the 4-5 seconds it takes the VAB to load and then just launch again without adjusting... many, many times.  I've yet to find a way to get those settings off by default.

I do this all the time, I’ll revert to fix some mission breaking flaw, tweak a bunch of other stuff, and launch again without fixing the actual problem. Sometimes multiple times in a row. 

[jimmymcgoochie]

Getting half way through a mission but having to stop for the day/game crashes, then when I reload later I forget I was half way through a mission and go to do something else instead. I’ve had plenty of Kerbals dead doing EVAs (with Kerbalism so limited life support on their suits runs out in a few hours) because I sent them out and then forgot they were there so time warped ahead for something else and the alerts kicked in too late…

[GuessingEveryDay]

Not enough EC for transmitting science. The worse part is that I keep forgetting the ratio, so I just assume the battery in the probe is enough.

[pandaman]

I voted 'Other'.

'Forgetting' to stop playing, and then ...

1. Getting into trouble with my wife.

2. Realise I'm hungry because I haven't eaten for far too long.

3. Almost wet myself because I just need 'one more' try.

4...

  Need I go on?  We've all been there.