Highlighted tile can "hesitate" between two tiles
OK, this bug is very visual but somewhat hard to describe in text, let's see what can I do with my non-native English...
Set the game to tiled mode. Place the cursor pointing to a tile, the tile will be highlighted with four golden arrows. Now move the cursor very slowly towards an adjacent tile.
If you do it right, there will be a point in this trajectory where the golden arrows "hesitate" between the two tiles, i.e., they repeatedly move very fast from one tile to another, creating a quite unpleasant effect. Although I mention the slow movement as an easy way to reproduce the bug, this can be triggered randomly during a game just from placing the cursor in the wrong place.
Note that I have observed the bug both in full screen and windowed mode, but at least in my machine it's triggered more easily in full screen (maybe because it has more resolution, the window is too small to locate the precise points that trigger the bug?), so I recommend full screen to reproduce.