Such as slabs. Clicking on the top puts the slab on the bottom, which makes the whole tower fall. I posted it on Reddit [https://www.reddit.com/r/QuarkMod/comments/ic1bw9/i_dont_think_scaffolding_replacement_should_work/ here] and Vazkii asked me to make a github issue. On the other hand, placing blocks on top of the tower by clicking the side is probably okay.