vSphere by default stores vswap files (the files used when your host runs out of physical memory to allocate) with the local machines. However, you can choose (on a per VM or per-host basis) to direct all swap files to one location.
VMware admins redirect swap files for many reasons – lower cost of local disk space, array-based reasons (snapshots, replication) etc – but not many of them know that the setting might change back to “keep with the virtual machine” automatically!
Here is an ESXi 5 host in a cluster. Since the arrays available to me are virtual/demo ones with very limited space I set the host to keep the swap files on the ESXi server’s “local” disk.
Ok, no problem! When I fire up or vMotion a VM to this host (assuming the vMotioned machine’s swap file isn’t on shared storage!) the swap file gets created on the local disk.
Now if I power cycle my VMs they won’t start up at all, until I either increase their reservations or change the swap file settings on either the VMs or host.