As an experienced cold water, poor vis, tidal wreck diver (and cave diver), who is now living in a place with warmer, better vis, but still current and deep... I would say whatever you are not experiencing in your local environment, for example:
- Underestimaging depth in ideal circumstances (warm, good vis water). Gas consumption, Gas density, narcosis, deco.
- Blue water descends/ascends without any reference
- Blue water ascends with reference
- Team/Buddy separation
I have seen it time and time again, people overestimating their own experience because their local diving is more "difficult". Yes for sure a north sea wreck dive can potentially be more difficult than a red sea wall dive or an italian wreck, but every environment has his own particular risks.
I've seen experienced divers not able to hold depth on ascend/deco stops, seen them lose ascend lines, seen them drift off in the wild blue yonder, seen divers overbreathing their regs at depth fighting current in clear vis, warm water getting close to a hypercapnia hit, I've seen divers totally underestimating depth...