This is a warning that a few days ago, a Java update versioned 1.8.0_20 was released, which currently makes it impossible to play modded Minecraft. The release fixed a Java bug which was hiding a bug in Forge Modloader- users with this new version of java will now receive a ConcurrentModificationException when trying to load modpacks in Minecraft 1.6+.
Forge is currently working to correct this issue, but in the meantime, we advise that you do not update your Java version to 1.8.0_20. We instead recommend that you use one of the following versions:
Java 1.7.0_67- we highly recommend that you downgrade to this version, because it is security complete: http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html
Java 1.8.0_11- if you need to stay on java 1.8, this is the latest version without the listed issue. Again, there may be security updates in 1.8.0_20 not present in this version, so please use with care: http://www.oracle.com/technetwork/java/javase/downloads/java-archive-javase8-2177648.html