With staircase blocks arranged in "staircase" pattern, such that there are two blocks on the bottom, and on block on top; standing next to one of the bottom blocks and looking "down" targets the block which should be hidden by the top block.

The screenshot is taken from a slight angle, to demonstrate the blocks involved, but this also happens when the lower-back block is completely hidden from view.

This happens when the cross-hair is pointing at the "back half" of the lower-front block. Pointing towards the "front half" of the block correctly targets the lower-front block.

I think I have seen this happen with non-staircase blocks (still arranged in a staircase pattern) but I am only able to consistently reproduce the issue using staircase blocks