Any excavated cave tile has a chance of caving in. When this happens, the tile is simply returned to its former non-excavated state and can be excavated again. If you mine a tile soon after it collapses, it won't need as many mine actions as usual to re-open. Ore veins may appear in cave-ins.
Reinforcing a cave wall with a [[support beam]] lowers the chance of cave-ins, but reinforced walls cannot be excavated, but they can be minedwhen found on a deed. After approx 50 shards mined from a reinforced wall, the reinforcement is removed and the wall can then be mined out normally. Ore veins are said to count as reinforced for purposes of stability.
Note: The only cave-ins/tile collapse that I have seen are those next to other unmined tiles.
For example: If you mine a three (3) tile wide path, the center tile won't cave in but the outter tiles will have a chance to cave in...
== Other considerations ==