When sneaking, the position of the center cursor and the actually selected block are misaligned.

For example, if you aim at the side of a slab, it will stick to the top of the slab. If you attach a hopper aiming at the side of the comparator, the hopper will attach to the top.
Attached is a picture with highlight enabled, sneaking and pointing the cursor at the wall, and the actual stair block pasted.