VanishNoPacket 3.19.1-SNAPSHOT, dev build 240.

When a staff member or player vanishes, their skin reverts to a steve skin. When they unvanish, it is back to normal. On Minecraft 1.7.9.

(Also, jenkins doesn't seem to be building anymore?)

See the TagAPI release notes, as I'm betting you're using TagAPI. Also, jenkins was taking a nap and is awake now.

Basic summary: 1.7.6, 1.7.7, 1.7.8 will disconnect the client if they receive skin data for someone who isn't ACTUALLY on the server (eg. skin data for a colored username, because the user on the server is the name without color). Mojang did not force updating to 1.7.9 which doesn't do that.

If I let TagAPI send this data, anybody who didn't update their client to 1.7.9 because they didn't realize they weren't on latest (I do this regularly, not noticing what version I'm on until a protocol change forces me to update) are going to get knocked off the server for a reason that confuses them.