Not so much to due with mining as building, but when I was building my tower near the height limit I didn't light it up because I was lazy and didn't think anything would spawn there. Shortly afterwards I turned around and saw multiple skeletons+zombies standing on the framework of the other side.
Also, while building the high wall around the base to keep mobs OUT, they continually spawned on top of the outermost later and proceeded to fall in and ambush me. (the tower keeps everything in permanent shadow)
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Also, while building the high wall around the base to keep mobs OUT, they continually spawned on top of the outermost later and proceeded to fall in and ambush me. (the tower keeps everything in permanent shadow)
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Simple things like new trees and the Nether as an actual explorable environment.