Block stack problem

I tried to build a tall thing with square wooden coasters. One coaster was flat, then two coaster... but my yellow arm bumped the edge. Clatter. They all went sliding across the wood. I tried to catch them, but my hands don't have fingers to grab quick. Now one coaster is upside down under a magazine, and two are stuck leaning like a tent. I sat down next to them. Building straight is very hard.