What this warn means? Its annoying to read this in console if its not a bug/problem

[21:43:18] [pool-3-thread-1/WARN]: Exception in thread "pool-3-thread-1"
[21:43:18] [pool-3-thread-1/WARN]: org.apache.commons.lang.UnhandledException: Plugin Vault v1.5.0-b28 generated an exception while executing task 3
at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:56)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NoSuchMethodError: org.bukkit.Server.getOnlinePlayers()Ljava/util/Collection;
at net.milkbowl.vault.Metrics.postPlugin(Metrics.java:387)
at net.milkbowl.vault.Metrics.access$400(Metrics.java:63)
at net.milkbowl.vault.Metrics$4.run(Metrics.java:275)
at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)
... 3 more

This is a duplicate issue of #586 . 1.5+ will not with with any versions lower than 1.8 as clearly stated. Update your spigot build to 1.8 if you wish to use the 1.5 version of vault.