Added warning in the multires panel to alert user if they have a modifier that changes mesh topology enabled, the render level will not have any effect.

source/blender/include/multires.h

@@ -52,5 +52,6 @@ void multires_update_levels(Mesh *me);
 void multires_level_to_mesh(Object *ob, Mesh *me);
 void multires_calc_level_maps(MultiresLevel *lvl);
 void multires_edge_level_update(void *ob, void *me);
+int multires_modifier_warning();
 
 #endif

source/blender/src/buttons_editing.c

@@ -4802,6 +4802,12 @@ void editing_panel_mesh_multires()
 
 			uiDefButC(block,NUM,B_NOP,"Render: ",cx,cy,200,19,&me->mr->renderlvl,1.0,me->mr->level_count,0,0,"Set level to render");
 			cy-= 20;
+			
+			if(multires_modifier_warning()) {
+				char *tip= "One or more modifiers are enabled that modify mesh topology";
+				uiDefIconBut(block,LABEL,B_NOP,ICON_ERROR, cx,cy,20,20, 0,0,0,0,0, tip);
+				uiDefBut(block,LABEL,B_NOP, "Cannot use render level", cx+20,cy,180,19, 0,0,0,0,0, tip);
+			}
 		}
 	}
 

source/blender/src/multires.c

@@ -39,6 +39,7 @@
 
 #include "DNA_mesh_types.h"
 #include "DNA_meshdata_types.h"
+#include "DNA_modifier_types.h"
 #include "DNA_object_types.h"
 #include "DNA_scene_types.h"
 #include "DNA_vec_types.h"
@@ -47,6 +48,7 @@
 #include "BKE_depsgraph.h"
 #include "BKE_global.h"
 #include "BKE_mesh.h"
+#include "BKE_modifier.h"
 
 #include "BIF_screen.h"
 #include "BIF_space.h"
@@ -1366,3 +1368,25 @@ void multires_edge_level_update(void *ob, void *me_v)
 
 	allqueue(REDRAWVIEW3D, 0);
 }
+
+int multires_modifier_warning()
+{
+	ModifierData *md;
+	
+	for(md= modifiers_getVirtualModifierList(OBACT); md; md= md->next) {
+		if(md->mode & eModifierMode_Render) {
+			switch(md->type) {
+			case eModifierType_Subsurf:
+			case eModifierType_Build:
+			case eModifierType_Mirror:
+			case eModifierType_Decimate:
+			case eModifierType_Boolean:
+			case eModifierType_Array:
+			case eModifierType_EdgeSplit:
+				return 1;
+			}
+		}
+	}
+	
+	return 0;
+}