I'm not sure what the problem is, but I can fix it...

Just put this after you draw the lemon:
PUT (50, 50), P(1, 5), PSET

If I had to guess, I'ld say a memory leak, but don't take that as an answer.

I know it is not a syntax error.

Just change the lemon code to:

'Lemon = $0
'Left Lemon
CIRCLE (137.5, 150), 30, 14
PAINT (137.5, 150), 14, 14
GET (50, 50)-(200, 200), p(1, 6)
PUT (50, 50), p(1, 6), PSET
PUT (50, 50), p(1, 5), PSET

It should run. But I'ld find the cause of this problem before I continued work on it.