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1) Sign loading code was trying to parse the JSON before the check for oldSign.
That code could then skip the old sign converting code if it triggers a JSON parse exception.
2) New Mojang Schematic system has Tile Entities in the new converted format, but missing the Bukkit.isConverted flag
This causes Igloos and such to render broken signs. We fix this by ignoring sign conversion for Defined Structures
Paper 
High performance Spigot fork that aims to fix gameplay and mechanics inconsistencies.
IRC Support and Project Discussion
Documentation
Access the paper docs here: paper.readthedocs.io
How To (Server Admins)
Paperclip is a jar file that you can download and run just like a normal jar file.
Download a copy of paperclip.jar from our build server, here.
Run the Paperclip jar directly from your server. Just like old times
Paper requires Java 8 or above.
How To (Compiling Jar From Source)
To compile Paper, you need JDK 8, maven, and an internet connection.
Clone this repo, run ./paper jar from bash, get files.
How To (Pull Request)
See Contributing
Special Thanks To:
YourKit, makers of the outstanding java profiler, support open source projects of all kinds with their full featured Java and .NET application profilers. We thank them for granting Paper an OSS license so that we can make our software the best it can be.