See now it's just what I said it would be. You'r not exiting the script after it jumps to a certain bookmark. The script swoops through all the bookmarks and the last titletext will be shown, in this case "5"
; Unit selection
_Unit = _this select 0
; Percentage generator
_number = Random 100
; Consequence selection
~5
?_number <= 35 : goto "WellDone"
?_number <= 45 : goto "1Left"
?_number <= 55 : goto "2Left"
?_number <= 65 : goto "3Left"
?_number <= 100 : goto "DeadPlan"
#WellDone
Titletext ["1","Plain down"]
exit
#1Left
Titletext ["2","Plain down"]
exit
#2Left
Titletext ["3","Plain down"]
exit
#3left
Titletext ["4","Plain down"]
exit
#DeadPlan
Titletext ["5","Plain down"]
exit