What happened was I was trying to start a snapshot server and I knew that starting it on the command line didn't work so I tried on the gui and it told me:
Invalid Session (try restarting your game)
so I tried and it didn't work again.

Crash report:
---- Minecraft Crash Report ----
// My bad.

Time: 2/22/14 2:12 PM
Description: Exception in server tick loop

java.lang.NullPointerException: Exception in server tick loop
at com.mojang.authlib.GameProfile.hashCode(GameProfile.java:90)
at java.util.WeakHashMap.hash(WeakHashMap.java:364)
at java.util.WeakHashMap.put(WeakHashMap.java:516)
at org.apache.commons.lang3.builder.ToStringStyle.register(ToStringStyle.java:191)
at org.apache.commons.lang3.builder.ToStringStyle.appendClassName(ToStringStyle.java:1453)
at org.apache.commons.lang3.builder.ToStringStyle.appendStart(ToStringStyle.java:378)
at org.apache.commons.lang3.builder.ToStringBuilder.<init>(ToStringBuilder.java:263)
at org.apache.commons.lang3.builder.ToStringBuilder.<init>(ToStringBuilder.java:226)
at com.mojang.authlib.GameProfile.toString(GameProfile.java:97)
at ov.d(SourceFile:102)
at ov.a(SourceFile:93)
at ok.c(SourceFile:130)
at net.minecraft.server.MinecraftServer.v(SourceFile:555)
at nb.v(SourceFile:264)
at net.minecraft.server.MinecraftServer.u(SourceFile:471)
at net.minecraft.server.MinecraftServer.run(SourceFile:387)
at mr.run(SourceFile:657)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– System Details –
Details:
Minecraft Version: 14w07a
Operating System: Mac OS X (x86_64) version 10.8.5
Java Version: 1.7.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 62625048 bytes (59 MB) / 102891520 bytes (98 MB) up to 1248985088 bytes (1191 MB)
JVM Flags: 0 total;
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Profiler Position: N/A (disabled)
Player Count: 0 / 20; []
Is Modded: Unknown (can't tell)
Type: Dedicated Server (map_server.txt)

Details:
you don't get a crash report on 14w08a but you still can't join