from Karl Tomlinson <k.tomlinson@auckland.ac.nz>, who writes:

There are two issues that are resolved here:

1 The call to xf86ReplaceStrOption in DeviceOn, when the device
  file can no longer be opened seems unnecessary and causes
  problems.  If Protocol is auto-dev then Device is reset anyway
  (through SetDeviceAndProtocol).  If Protocol is something else
  then the device file will never be found even after it is
  recreated because Device is never set again.

2 The first xf86OpenSerial in DeviceOn may successfully open a
  different mouse (or other event file) that has the same name as
  the Device found through the first invocation of auto-dev.  It
  seems reasonable that if auto-dev is used to find the device
  file the first time, then it can be used to find it again after
  the device has been closed (and possibly changed name).