What might happen if a SCSI bus is not properly terminated?

Performance may suffer; the initiator (SCSI controller) may not see the SCSI targets (i.e, disks) ... or they may disappear at some really bad time; or performance might be great but you will get a lot of errors and data corruption ; The correct answer depends on a lot of variables including cable length; whether or not the controller and/or any of the disks are terminated; type of SCSI you use; quality of SCSI cabling; and SCSI device physical topology. Your computer may not even boot or spin up the whatever is connected to the SCSI controller. Bottom line, SCSI absolutely must be terminated properly. If it isn't, bad things may happen at unpredictable times.