This happened again when he was about nine months old. No obvious factors (sick or lack of sleep), but I recognized the crying. I was alone this time while my wife was away on business travel (so every night is "my turn"). I watched him via the monitor and saw that he was "playing" with his crib mounted Fisher Price Aquarium (a new favorite nighttime toy). He was like a zombie, hitting the toy (lights and music) then crawling around the crib, lay down for a minute, then pop back up and hit the aquarium again. This went on for 15-20 min and I decided to intervene. This toy was overstimulating him during this "stuck REM cycle" and perpetuating the night terrors.
Once in the room, I had to pull the toy off (not easy as it was strapped and tied on), so this was really going to mess him up. I picked him up and held him close while I rocked in the chair, humming some tunes from The Doors. (see, dads are cool) After about 20 min he was sleeping and I gently put him back down. Of course it wasn't quite that easy, it took one more round of rocking to make sure he was down for the night. This toy was not responsible for the terrors, but he was on zombie-autopilot when they kicked in...so the toy had to be put away.
No issues since....kow