So, I used Minecraft Dev 0.9 yesterday, and it worked fine (I exit and played again, so it wouldn't have been a really big bug yesterday) and now, I kept on clicking Play, and I've noticed something. I just deleted everything related to minecraft, downloaded minecraft first, and then tried Minecraft Dev, but it kept on happening. It kept on saying:
net.minecraft.client.main.Main --username Ambamore2000 --session 8cd1a644354ab6e1719da06275ac17ab655c7184 --version 13w23b --workDir "/Users/Janice/Library/Application Support/minecraft"
---- YOU CAN CLOSE THIS LAUNCHER IF THE GAME STARTED OK ----
---- YOU CAN CLOSE THIS LAUNCHER IF THE GAME STARTED OK ----
---- YOU CAN CLOSE THIS LAUNCHER IF THE GAME STARTED OK ----
---- (We'll do this automatically later ;D) ----
Client> Exception in thread "main" java.lang.NoClassDefFoundError: net/minecraft/client/main/Main
Client> Caused by: java.lang.ClassNotFoundException: net.minecraft.client.main.Main
Client> at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
Client> at java.security.AccessController.doPrivileged(Native Method)
Client> at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
Client> at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
Client> at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
Client> at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
Game ended with bad state (exit code 1)
Deleting /Users/Janice/Library/Application Support/minecraft/versions/13w23b/13w23b-natives-1370887198502822000

at the end. I hope this gets fixed, because I tried 3 times.