Problem is this. I described on title. Sorry for bad English.

What plugins do you use?

What mc version do you use?

Server Version : 1.8.8 paperspigot

Plugins :
AAC alphachest antijoinbot antispam auctionhouse authme autorank changeskin chestcommands chestshop citizens clearchat clearlag combattracker deathmessages essentials factions featherboard frameprotect groupmanager hideandcustomplugins holographicdisplays iconomyrecreation namebugfixed itemcases letitrain lockette massivecore mineresetlite multiversecore mvdwplugin mythicmobs plotsquared protocollib pluginmetrics scheduledannouncer2 serverlistmotd shopkeepers simplevipperks stats tabprefixes timtheenchanter titlemanager vault worldedit worldguard

what authme version do you use?

i think 3.5 snapshot

Could you make a "/protocol dump" ?

http://hastebin.com/cuyotawusa.sm

And when does it happen? Is a different inventory open in the meanwhile?

i didnt see that problem. A player of our server said. A reliable player.

I think inventory of the skin he changed to opened.

Which build of PaperSpigot do you use?

This server is running CraftBukkit version git-PaperSpigot-"8b18730" (MC: 1.8.8) (Implementing API version 1.8.8-R0.1-SNAPSHOT)

Could you try this one: https://github.com/games647/ChangeSkin/releases/tag/inventory

please?

I think, it's fixed. And thanks for your help, sorry for bad English. I want to all developers to help me like you :)

Looks like it actually was a bug in Spigot

see here:
https://hub.spigotmc.org/stash/projects/SPIGOT/repos/spigot/commits/1e4dd71cf216f953a00e1a82901703b39b84ee6e