These messages do not prevent mining but inform the player of the result of their actions should mining continue. These may be intentional or otherwise.

The cave walls sound hollow.
Action: Mining a tile wall.
Cause: Height difference between shafts.
Fix: You can continue mining.

Mining will be no longer possible when receiving one of the following messages.

The cave walls sound hollow. A dangerous side shaft could emerge. [The cave shudders from some imbalance.]
Action: Mining a tile wall.
Cause: Tile shares a single corner with an existing open area.
Fix: Mine adjacent tiles (go around).

Another tunnel is too close. It would collapse.
Action: Opening a tunnel from the surface.
Cause: Another tunnel is keeping you from tunneling in.
Fix: Try another tile.

The cave walls look very unstable. You cannot keep mining here.
Action: Mining a tile wall.
Cause: Ceiling too close to rock layer above.
Fix: Try mining downward or lowering the mine floor.

The cave walls look very unstable and dirt flows in. You would be buried alive.
Action: Mining a tile wall.
Cause: You're close to the surface, about to break through, but it's stopping you because dirt is covering the surface where you would open an exit.
Fix: Dig to rock above on the tile and surrounding tiles, then try again.

A dangerous crack is starting to form on the floor. You will have to find another way. [The cave shudders from some imbalance.]
Action: Mining a tile wall.
Cause: A tunnel exists above (dropshaft).
Fix: Work from the high side.

The ground is too steep to mine at here. You need to make it more flat.
Action: Mining a tile floor.
Cause: Too steep.
Fix: Go with less steep slope.

The ground sounds strangely hollow and brittle. You have to abandon the mining operation.
Action: Opening a tunnel from the surface.
Cause: Already a tunnel on the tile being tunneled.
Fix: Choose another tile.

The topology here makes it impossible to mine in a good way.
Action: Creating (tunneling) a mine entrance.
Cause: Flat rock tile.
Fix: Lower one corner of the entrance tile by at least 1 slope so that it is not flat.

The mountainside would risk crumbling. You can't open a hole here.
Action: Creating (tunneling) a mine entrance.
Cause: Rock tile is too steep. Max slope is 270. OR there is a mine under where you're trying to open.
Fix: Lesser slope for the entrance, or a different location not above a mine.

There is no space to mine here. Clear the area first.
Action: Mining.
Cause: Max items (100) on a tile.
Fix: Clear the tile of items.

You fail to produce anything here. The rock is stone hard.
Action: Mining a tile on the surface.
Cause: You're mining an ore tile.
Fix: Pick another tile.

You hear falling rocks from the other side of the wall. A deep shaft will probably emerge.
Action: Mining a tile wall.
Cause: Another mine nearby of great height difference.
Fix: Go around, redesign. You can't connect the two because of the height difference.

You cannot keep mining here. The rock is unusually hard.
Action: Creating (tunneling) a mine entrance.
Cause: The tile is an ore vein.
Fix: Pick another tile.

The roof sounds strangely hollow and you notice dirt flowing in, so you stop mining.
Action: Mining a tile wall.
Cause: Mine ceiling is too close to the surface. Rock level is most likely very close to a thin layer of the dirt on the surface.
Fix: Raise the surface rock level or mine downward.

The water is too deep to mine here.
Action: Mining a tile floor.
Cause: You cannot mine past -25. On Mythmoor the lowest passable level for wagons is -6 and -14 for carts. All boats can float and move in -5 of water.
Fix: Stop at the limit or fill in the file and pick another level above the limit, then mine to that.

The roof sounds strangely hollow. You must mine the other corners if you want to create an opening.
Action: Mining a tile ceiling.
Cause: Ceiling height and surface rock height are the same.
Fix: Raise corners to create an opening, or stop mining.

The roof sounds dangerously weak and you must abandon this attempt.
Action: Mining a tile wall.
Cause: Rock layer is close to your mine ceiling.
Fix: Stop mining, try mining downward or choose another path.

You cannot reach the ceiling.
Action: Mining a tile ceiling.
Cause: Ceiling is too high.
Fix: Build a temporary building so you can reach the ceiling and mine higher.

You are not allowed to mine here.
Action: Mining.
Cause: Deed permissions do not allow you to mine.
Fix: Talk to the mayor.

The rock sounds hollow, you will need to tunnel to continue.
Action: Surface mining.
Cause: Mining above an open mine tile and reaching the ceiling height of that mine tile from the surface.
Fix: Stop mining. Make desired adjustments.

The rock is too hard to mine.
Action: Mining a tile wall.
Cause: Reinforced tile (or stacked actions that can't mine after a mine wall falls).
Fix: Reinforcement can only be removed when they are in the boundary of a settlement.

You are too unskilled to mine here.
Action: Surface mining.
Cause: Skill level not high enough.
Fix: Skill up a bit in mining.

An altar of (deity) on the surface disturbs your operation.
Action: Mining.
Cause: An altar is on the tile you are trying to mine.
Fix: Push the altar to another tile.

This tile is protected by the gods. You can not mine here.
Action: Mining.
Cause: GM protected tile.
Fix: Put in a /support ticket to have a GM assess the tile and possibly remove its protected status.