I made two rail systems that are parrallel but independant from each other. On the corners, the carts can go a block offset from their rails for a moment (sometimes for a prolonged time).

Initially i though it was only a graphical issue, but apparently, if the cart gets offsetted on the other rails' side, it can sometime remain there. Long story short, on the corners the carts can move from their rails to the parallel rails.

I dunno how to better explain this, hopefully i'm not the only one here...

This bug was present on the previous snapshot as well.