This bug has been found in r11372.
When a tunnel building tool is selected, it creates a tooltip that shows the estimated length and cost of a tunnel that would be built where it is, or "No suitable ground" if such a tunnel can't be built.
When sliced underground mode is active, there are some tiles that are underground (they have a black background) and some that are not. When the tunnel builder tool is moved to another, the tooltip is supposed to be changed according to tunnel building possibilities on the tile it is moved on. However, moving the tool to an underground tile does not change the tooltip. This means if the tool is e.g on a slope, and the tooltip displays "Length: 8, Estimated construction cost: 200.00$", then when I move the tool to an underground tile, the tooltip will still display "Length: 8, Estimated construction cost: 200.00$".
tunnel-tooltip.gif
Since tunnel building tools are used to build tunnels in (sliced) underground mode in much the same way as regular track tools are used in non-underground areas, I think having a tooltip when the tunnel building tool is on an underground tile would be useless and intrusive. Thus I suggest that, when the tunnel tool is moved to an underground tile, it clears its tooltip.