Most flagrantly this can be seen with Wallwalker (Banshee/Dread/Xorn/Umberhulk/etc). They become aware of you, and just amble about aimlessly in the walls. The usually gradually approach you via a series of circles and backtracks that takes hundreds of turns over a direct route in some cases. Sometimes, they just kind of amble away, puttering around in a corner and never approaching. Found a perfect example to illustrate this.
This Umber Hulk started at the square marked in red. I tunneled to where my guy is currently standing, at which point he became aware of me. Then, according to my combat log, over the next 64 turns while I rested, he ambled about digging out that huge area you can see below him before finally approaching me. This is an issue with all Wallwalkers currently.
There's also some subtler issues, and I'll throw up a pic next time it happens. This has to do with enemy boss/rare AI (particularly casters). Good example is The Master. I killed him this time by standing out in the open around a corner, and just casting slow such that it hit him out of my LOS. He just wandered back and forth between two squares, never approaching me or doing anything.