Hmmm, that's actually pretty hard because I rarely get frustrated or all "Oh no, you don't" unless the conduct feels forced and serves more plot than character. My dislike of something in the show pretty much always comes from a Doylist place.
Instead, since I'm a Scooby stan, I pick, in quote form, one conduct from each Willow, Xander, Giles, and Buffy in each season that didn't take me out of the story and tells something dysfunctional about the involved character(s) and interpersonal Scooby dynamic. Not technically 'screw-ups' like mindwiping or feeding friends to demons but conducts that carry, among other things, also negative connotations, sometimes quite upfront, sometimes more subtle and sometimes even misleading in varying degrees of severity. I leave them without commentary.
Season One:
Buffy: "Thanks! Boy! Cordelia's hips are wider than I thought!" - Never Kill a Boy on the First Date
Willow: (to Buffy) "Malcolm said you wouldn't understand." - I Robot, You Jane
Xander: (to Willow) "Hey, I know what we'll do! We can go! Be my date! We'll, we'll have a great time! We'll dance, we'll go wild... Whadaya say?" - Prophecy Girl
Giles: (to Willow & Buffy) "Um, this computer invasion that Willow's performing on the coroner's office, one, one assumes it is entirely legal?" [...] "Right. Wasn't here, didn't see it, couldn't have stopped you." - Teacher's Pet
Season Two:
Buffy: (to Willow and Xander) "Look, this is Slayer stuff, okay? Could we have just a little less from the civilians, please?" - When She Was Bad
Willow: (to Oz) "Do you wanna make out with me?" - Innocence
Xander: (to Buffy) "No." (at Buffy's inquiry if he and Willow had fun over the summer) - When She Was Bad
Giles: (Buffy to Willow) "You know what the worst thing is? I was saving up for some very important shoes, and now I have to blow my entire allowance to get this stupid tattoo removed. Let's just hope my mom doesn't see it first." - The Dark Age
Season Three:
Buffy: (to Willow) "But... there's a 'but'. And that's 'but you shouldn't come... tonight.' Is that cool?" - Bad Girls
Willow: (to Xander) "No! Of course not! This is a purely scientific... ...de-lusting spell... for us. I thought it would go better if you didn't know." - Lover's Walk
Xander: (to Buffy) "Look. I'm sorry that your honey was a demon, but most girls don't hop a Greyhound over boy troubles." - Dead Man's Party
Giles: (Willow) "Nothing... much. Well, you know, I tried this spell to cure Angel, and I guess that was a bust. But since then, you know, small stuff: floating feather, fire out of ice, which next time I won't do on the bedspread. Are you mad at me?" (Giles) "No, of course not, no. If I were, I would be making a strange clucking sound with my tongue." - Faith, Hope & Trick
Season Four:
Buffy: (to Willow and Xander) "Judgmental? If I was anymore open-minded about the choices you two make my whole brain would fall out!" - The Yoko Factor
Willow: (to Xander) "Oh, like what? "Oh, poor me.. I live in a basement. Yeah, that's dire." - Something Blue
Xander: (to Willow) "Will, not loving the drowning of the sorrows." [...] "Right. Topic now. Will, how about you give me that beer?" [...] "We all have pain, Will." - Something Blue
Giles: (drunk, to Buffy) "You never train with me anymore. He's gonna kick your ass." - The Yoko Factor
Season Five:
Buffy: (to her friends.... and Spike): "Remember, the ritual starts, we all die. And I'll kill anyone who comes near Dawn." - The Gift
Willow: (to Xander) "Sometimes we all helped save you." - The Replacement
Xander: (to Spike) "Because, Willow's my best friend and I really value her opinion, [...]" - Triangle
Giles: (Xander) "There. How you doing?" (Giles) "Only hurts when I answer pointless questions." - The Weight Of The World
Season Six
Buffy: (to Willow) "I get that. More than you- But it's wrong. People get hurt." [...] "Good. I think it's right. To give it up. No matter how good it feels." [...] "Exactly. It's over." - Wrecked
Willow: (to Buffy) "The hard way. The spell-free way. The oh-my-god-my-head's-gonna-fall-off, my-feet-are-killing-me way. I don't know how I got through this day." - Gone
Xander: (to Willow) "Excuse me? Who made you the boss of the group?" - Bargaining I
Giles: (to Buffy) "I had so hoped it wasn't her [Willow]." and "Sometimes the most adult thing you can do is ask for help when you need it." - Grave
Season Seven
Buffy: (Willow) "[...] Mine said it came with a message from Tara.[...]" (Buffy) [...] "A Vampire I killed last night told me Spike sired him...two nights ago." - Sleeper
Willow: (to Buffy) "Aw, it's all right. You needed to be. Although, Twinkies and kisses? Also peachy motivational tools. You OK?" - Get It Done
Xander: (to Buffy) "When? Just now? Took you all of ten seconds to decide to kill one of your best friends?" - Selfless
Giles: (to Buffy) "It's time to stop playing the role of general, and start being one." - Lies My Parents Told Me