I don't know if that's really a bug or a wanted feature, but there are some items that stack with skills and others that don't stack.

I tried it with several items on my server and i'm unsure about these 3:
-Mask of Death DOES stack with Vampiric Aura from Undead Scourge. I found out that max leech possible is 40 (if(leechhealth>40) leechhealth=40; // woah, woah, woah, AWPs!) but with Mask + Vampiric you can leech 2*40 in one hit.
-Sock of the Feather DOES stack with Levitation from Undead Scourge. I don't know if the gravity decreases are added and then applied to the normal gravity or the item takes the reduced value from the skill and reduces it again. The decrease from the item didn't seem to be as much as if the item is used without the skill.
-Boots of Speed DO NOT stack with Unholy Aura from Undead Scourge. I ran quite a long distance once with only the aura and once with aura + boots, no difference in the needed time.

These 2 seem to be alright:
-Ring of Regeneration + Shadow Hunter DOES stack. Didn't test with the shopmenu2 regeneration items or other regeneration races...
-Periapt of Health + HP Buff Skills DOES stack. Didn't test it again but I remember that from playing and spectating. Not sure about all races though...

I didn't try other items (like Cloak of Shadows + Human Alliance needs at least 2 persons, ...) or other races but I think that you can check that by looking at the code of the items and races. If you want me to try some special combination or add a replay I can do that.

If these are no bugs, please give me a short list of which items can stack with which skill =)
I hope I don't annoy you by reporting this much Better report/ask than playing with possible bugs, right?)

Regards

it depends on what type of buff the race uses.

you really have to look at War3Source_Engine_BuffSpeedGravGlow to see how they are calculated

the ones you listed are intentional.
and you can ban those items on those races.

Vulpone, I test those stuff on a private password server, so no problem there. I use sv_cheats to test stuff quite often but didn't know about a thirdperson in cs:s, thx for that =) I sat there and thought "...want to test human invis and cloak ... need thirdperson..." but didn't look for a command.

Ownz, I will have a look at War3Source_Engine_BuffSpeedGravGlow, hopefully I can understand enough.
And I forgot about being able to block items for races, that's right, thx for reminding me