I made a thread about this a couple of weeks ago.
In a nutshell, I got interdicted by an Anaconda which I flat-out couldn't damage with my Cutter.
My Cutter's got a C4 beam laser and a pair of C3 beam lasers and an attack run against this Anaconda would take a ring off it's shields and then, in the time it takes me to turn around for another attack, it would regenerate the shields to full.
Which happened about a dozen times.
I realised this could be down to SCBs, assuming the Anaconda was fitted with a whole heap of them, and decided to go "all in" with my kinetic weapons too.
I managed to strip his shields (briefly causing me to celebrate) but, once again, in the time it took me to get back on-target, they were back on line again and back up to full strength.
I'm pretty sure I've seen similar behaviour from smaller ships too, although I've been able to destroy them in my Corvette.
To put this in perspective, my 'vette can take out an Elite NPC Anaconda in about 30 seconds - strip shields with initial attack, switch to kinetics, manoeuvre around target, finish off by applying frag-cannons to soft, chewy centre and boom.
At the Lambrayeque HazRes, during the recent CG, I encountered a variety of NPC ships like AspXs, Cobras and Adders which took much longer than expected to go boom.
I'd be sitting there, unloading my C4 beams on them and they'd just soak it up for about a minute before finally losing shields and then being destroyed.
And, if they could get out of my line of fire, they'd immediately be back to full shields.
On one notable occasion, I found another Anaconda which seemed to have this ability but it decided to flee rather than fight.
So, I'm trundling along behind it, unloading my C4 lasers into it until, eventually it's shields go down, I'd then switch to kinetics and do a bit of hull damage but then I'd drop out of range.
At that point, I'd have to transfer power to thrusters, boost to get back in range and then it's shields would be back up to full and I'd have to start again.
In the end, I pursued it around 200km (which, at an average speed of around 170m/s gives you an idea of how long this went on for) before I managed to destroy it.
Overall, I kind of like that this happens because it forces the player to make more diverse choices and reduces complacency, especially when one of these ships is part of a wing, but it definitely seems like there are some NPC ships out there with extraordinary shields.