There used to be loads, but through various means of practice i've pretty much got them nailed;
Daytona road turn 1: Ran WRS here, learnt the corner. Still requires a couple of laps to get used to in a new car, but I know the corner quite well.
TGTT turn 1: When doing the SLS challenge here I learnt the fastest way around. The same line/approach even translates well over to Forza when I did a time trial event at Game in the Cee'd.
La Sarthe, the left hander just after the Porsche curves: I always ran wide here. But again, running the WRS here showed me that you need to get off the power and turn in much earlier, and don't shift up.
La Sarthe Mulsane (sp?) hairpin: I never used to be able to do this right. But a 24 hour race sorted this out. This occasionally hit the gravel but only when pushing the limits in the braking zone.
All the hairpins at Tsukuba: When doing the supercar seasonal event I learnt how to nail all of these, I never found them difficult it's just I was only simply matching the AI, I needed to find a faster way. Hugging the apex is pretty important here.
I think perhaps the esses at High Speed Ring are still a problem for me, I never seem to nail the entry to the first one. Blanchimont at Spa is also difficult, although I only drove this for the first time in GT5 and my only knowledge of the track comes from F1, and Blanchimont is flat in one of those.