Answer:

' Draw 100 circles, each in a new color
' (buggy program)
'
SCREEN 12
'
LET HUE = 4
LET COUNT = 1
'
DO WHILE COUNT <= 100
'
  LET HUE = HUE + 1
  COLOR  HUE
  CIRCLE (320, 240),50
  SLEEP 1
'
  LET COUNT = COUNT + 1
LOOP
'
END

What! A Bug?

The program is nearly OK, but it has a bug.

Each time the loop body executes, HUE is increased by one, the current pen color is set to HUE, and the new circle is drawn.

But look what happens when HUE gets up to 15:

  LET HUE = HUE + 1    ' HUE changes from 15 to 16
  COLOR  HUE           ' pen color set to 16
                       ' but there is no color 16---
                       ' CRASH!!
  CIRCLE (320, 240),50
  SLEEP 1

The program will work fine for a while, but since each execution of the loop body increases HUE by one, HUE will get too big for the system to handle long before the 100 circles have been drawn.

QUESTION 16:

(Thought Question:) Instead of changing HUE to 16, what else could you do?