Just answered it on Miiverse, but just to have the answer here as well:
LOAD automatically resizes the array to the size of the file. Your save file seems to be less than three values long, so it shrinks and SAV[2] is no longer valid.As for fixing it, I guess a simple way you might do it is something like this:
DIM WINS,TUTORIAL,SAV[5],SELECT 'Load file if it exists IF CHKFILE("DAT:SAVE") THEN LOAD"DAT:SAVE",SAV,FALSE ENDIF IF LEN(SAV)!=5 THEN PRINT "Invalid save file" STOP ENDIFSomething more advanced might try to recognize save files from older versions of the game and convert them; a good way of doing this is to have a version number at the beginning of the save file.