How to reproduce:
1. group some non-empty layers in a layer group
2. select the layer group
3. goto to 'Filters' menu and select one filter 

This should happen:
If filters are able to work on layer groups, they should behave like on a single layer: open the filters dialog, after user confirmation apply the filter. 
If filters are not able to work on layer groups, disable these particular menu items until they work.

This happens:
The filter dialog appears, the preview works on the composed layer group, but after user confirmation the following error message is shown: 'Calling error for procedure '<the filter procedure>' Item 'Layer Group' (3) cannot be modified because it is a group item'.
In case of the 'Distorts/Curve Bend' filter: GIMP shows the messages: 
'Plug-In "curve-bend.exe"
(C:\Program Files\GIMP 2\lib\gimp\2.0\plug-ins\curve-bend.exe). tried writing to a group layer 3 (killing).Plug-In 'Curve Bend' left image undo  in inconsistent state, closing open undo groups.' -> the filter leaves a temporary layer group in the image, which is of no use. 

It's confusing to the user to see that the filters work in a preview, but refuse working after confirmation.

It happens always. 
It's similar with Tools/GEGL ops: The enabled menu item makes the user believe this would work. Clicking on the item doesn't show up the GEGL dialog or anything else -> disable this menu item as long as it doesn't work.