Oftentimes the server can get spammed with warnings along the lines of:

[11:54:24] [Server thread/WARN]: Tried to add entity Sheep with pending removal and duplicate UUID d361162c-e5ef-41e8-8df9-741688e73c1a
[11:54:24] [Server thread/WARN]: Tried to add entity Villager with pending removal and duplicate UUID 44a5ac63-1215-4667-bed3-bdcde0055696

It appears that the root cause of these warnings is a delay of a tick (or maybe more) between entity removal, during which time a required chunk may be reloaded.

I have reproduced this in 14w49b Vanilla by flying around in creative at normal speed, but for a faster reproduction, the following may be helpful:

/give @p minecraft:golden_boots 1 0 {AttributeModifiers:[{AttributeName:"generic.movementSpeed",Name:"generic.movementSpeed",Amount:10,Operation:1,UUIDLeast:894654,UUIDMost:2872}],display:{Name:"Speed Boots",Lore:["Put on for a 10x Speed boost!"]}}

.

It is likely this issue also affects 1.8.8 but is simply unnoticed as the warnings have only been added in the current snapshot series.