When using a custom free floting toolbar the tooltip (the yellow box by the mouse pointer) sometimes displays behind the toolbar or other floating windows boxes (like Layers or Materials), see image.
Sketchup Pro 2015 v15.0.9350 64-bit (TRIAL)
Windows 7 Enterprise SP1
This happen randomly when moving/opening the toolbox or when moving/opening other free floting window boxes.
Create a new cusom free floating toolbar. Put some tools in it so its size is at least 2x2. Hoover the mouse pointer over a tool in the box, the yellow tooltip box is then sometimes displayed under the toolbox and/or under other floating windows.
If this does not happen then try again after moving the custom toolbar or after moving/opening other free floating windows or toollboxes.