When writing blockmodels, Minecraft suddenly rejected my resource pack and used the fallback one (default).
From the "Game Output" tab in the launcher:

[15:24:27] [Client thread/INFO]: Caught error stitching, removing all assigned resourcepacks
java.lang.NullPointerException
at bot.a(SourceFile:525) ~[1.8.8.jar:?]
at bot.g(SourceFile:421) ~[1.8.8.jar:?]
at bot.j(SourceFile:537) ~[1.8.8.jar:?]
at bot.a(SourceFile:139) ~[1.8.8.jar:?]
at bou.a(SourceFile:23) ~[1.8.8.jar:?]
at bnn.c(SourceFile:104) ~[1.8.8.jar:?]
at bnn.a(SourceFile:92) ~[1.8.8.jar:?]
at ave.e(SourceFile:652) [1.8.8.jar:?]
at ave$8.run(SourceFile:2325) [1.8.8.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_60]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_60]
at g.a(SourceFile:44) [1.8.8.jar:?]
at ave.av(SourceFile:881) [1.8.8.jar:?]
at ave.a(SourceFile:325) [1.8.8.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:124) [1.8.8.jar:?]
[15:24:27] [Client thread/INFO]: Reloading ResourceManager: Default

This very descriptive error was produced, because in one of the modelfiles, I put a "," at behind the last element of an JSON array.
(Happens only in the elements tag! When putting a comma behind one of the numbers in the from or to tag, it just says expected 3 values, found 4)

Sometimes (not always) this error even crashed the launcher running in the background. (v1.6.44)