Hmm, problem:
Error: Invalid subsystem name.
In sexpression: ( when
( has-arrived-delay 6 "Cypher" )
( fire-beam
"Cypher"
"turret12"
"Ithaca"
)
( fire-beam
"Cypher"
"turret20"
"Ithaca"
)
)
(Error appears to be: turret12)
File:E:\Languages\Visual Studio Projects\Visual C++\fs2_open\code\Mission\MissionParse.cpp
Line: 4539
Call stack:
------------------------------------------------------------------
------------------------------------------------------------------
Encountered it just after finishing the mission where the derelict first appears, if that's any help.