For me, it was discovering that while diving a 'normal', light, fun, recreational (non-skills/drills) dive, I have reasonably good buoyancy and trim........... until I get task loaded. Then I seem to gently sink into the sunset. My DIR-F instructor was seriously concerned that perhaps I didn't have good vision underwater as he'd only ever seen "bottom-crawling" tendencies in divers who weren't wearing vision correction (I was)............. I could maintain perfect buoyancy and hovers.........
six inches off the nearest surface, be it horizontal or vertical, and it's like I was locked into that position - I'd follow any contours anywhere never straying further than 6" off them.
That made the ascent drills just loads of fun.
Turns out it wasn't vision-related at all - it was a comfort issue with me being 'terrified' of an out of control ascent due to not being able to see the surface 'behind' me when properly trimmed out.
Okay, I've done a fair bit of work on that issue and can now confidentaly hold mid-water stops.... without being 6" from the nearest surface.
I was pleased to report this to my instructor recently and he sounded pleased as well.
That's been my biggest hurdle.
And yes, my bottom crawling tendencies only enhanced the humor factor of the post-dive video review........... man I thought the lot of us were gonna pee outselves from laughing so hard!!! The instructor could've made money on admission!