Slave.remoteFS is intended to be an absolute path.

(Could consider throwing IAE if it does not appear to be.
Cannot easily call getRootPath().absolutize() since that requires a connection.)

core/src/main/java/hudson/model/Slave.java

@@ -91,7 +91,7 @@ public abstract class Slave extends Node implements Serializable {
     private final String description;
 
     /**
-     * Path to the root of the workspace
+     * Absolute path to the root of the workspace
      * from the view point of this node, such as "/hudson"
      */
     protected final String remoteFS;
@@ -412,6 +412,10 @@ public abstract class Slave extends Node implements Serializable {
             if(value.startsWith("\\\\") || value.startsWith("/net/"))
                 return FormValidation.warning(Messages.Slave_Network_Mounted_File_System_Warning());
 
+            if (!value.startsWith("\\") && !value.startsWith("/")) {
+                return FormValidation.error(Messages.Slave_the_remote_root_must_be_an_absolute_path());
+            }
+
             return FormValidation.ok();
         }
     }

core/src/main/resources/hudson/model/Messages.properties

@@ -242,6 +242,7 @@ Slave.Launching={0} Launching slave agent
 Slave.Network.Mounted.File.System.Warning=Are you sure you want to use network mounted file system for FS root? Note that this directory does not need to be visible to the master.
 Slave.Remote.Director.Mandatory=Remote directory is mandatory
 Slave.Terminated={0} slave agent was terminated
+Slave.the_remote_root_must_be_an_absolute_path=The remote root must be an absolute path.
 Slave.UnableToLaunch=Unable to launch the slave agent for {0}{1}
 Slave.UnixSlave=This is a Unix slave
 Slave.WindowsSlave=This is a Windows slave