For me the worst is kits where you have to drill holes in something before putting together, especially with certain manufacturer aircraft and ship kits
This seems to be my Kryptonite
The first time I really got burned was with my 1/72 Academy F-14. I was coming off decades of building nothing but Games Workshop 40K and Bandai, Fine Molds, and Revell Star Wars stuff so it never dawned on me that a kit would actually need you to drill holes in it out of the box. Of course once I put the fuselage together I was missing all the holes for the armament wracks and had to wind up doing a best guess where they went and cutting off the pins and gluing flush
So I tried to be more careful after that, but still miss the occasion hole here or there as I sometime build out of instruction order due to painting requirements.
Most recently I was working on a Trumpeter 1/700 USS Yorktown and you need to cut some tabs so later you can plug in gun islands.
I had gone back and forth over the instructions and thought I caught them all, but then looking at the sprues,
I realized I had some parts that needed a place to go. And worse yet, it was one area of the hull that had hangar sections on the inside so you could no longer see the inner wall to find the recessed part that you need to cut out
So, I poured back over the side wall instructions again and sure enough found what I had missed.
My saving grace was that if I held up a really bright light at just the angle I could see the sink marks on the surface where the tabs should be and was able to rescue myself using that as a guide instead
But I think my worst moment was after putting lots of effort in painting a figure, I went to dull coat it, and instead blasted it with Black Primer from the same manufacturer. It did not dawn on me at first why the figure was getting darker with these little black spots all over after doing a few passes
I did the passed more like a mist coat so I was holding the can far away and moving quickly so it wasn't completely opaque black, but more misted black
I tried fixing it up, but of course I felt it never came out as good as it did the first time
If you look close at the skin area around his belly button you can still see a blackish tint to the skin
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