When placing blocks while crouched, the block often misplaces from where you aimed. This happens most frequently with placing slabs against a full block or with blocks such as observers etc.

When placing blocks while crouched the aim is off causing the block to place incorrectly. If you aim at the top of a full block and try to place a slab while crouched, it will often place above the block space even if you have aimed correctly at the top half of the full block you are trying to place the slab against.