on a dedicated server if you remove a mod and readd it it makes all the blocks in the mod invisible even blocks placed after the readdition. they are there but they cant be seen or interacted with. this only seems to be a problem on a dedicated server. if you do the same procedure on a client instance it works as intended.
tested with forge 1751, thaumcraft 5.1.6, and hydraulicraft 3.1.65.
Update: i just retested on 1752 and the issue is still there. if you load the world in single player none of the blocks are invisible but if you take it back to the server the bug returns
i believe this is a issue with the newish feature of forge remembering previously registered blocks even after a mod is removed
client log from instance joining server before removal after removal and after readding:
http://pastebin.com/w8Aeiwek
Server log from after readding. i can get the other 2 if needed
http://pastebin.com/iLfA046N