Assuming that I understand correctly (and that's a big
assumption), and starting with the stack "cleared":

keystroke Classic RPN stack RPL command line RPL stack
3 T: 0 3 (empty)
Z: 0
Y: 0
X: 3
ENTER T: 0 N/A 1: 3
Z: 0
Y: 3
X: 3

1 T: 0 1 1: 3
Z: 0
Y: 3
X: 1

+ T: 0 N/A 1: 4
Z: 0
Y: 0
X: 4

4 T: 0 4 1: 4
Z: 0
Y: 4
X: 4

ENTER T: 0 N/A 2: 4
Z: 4 1: 4
Y: 4
X: 4

3 T: 0 3 2: 4
Z: 4 1: 4
Y: 4
X: 3

+ T: 0 N/A 2: 4
Z: 0 1: 7
Y: 4
X: 7

X T: 0 N/A 1: 28
Z: 0
Y: 0
X: 28

2 T: 0 2 1: 28
Z: 0
Y: 28
X: 2

ENTER T: 0 N/A 2: 28
Z: 28 1: 2
Y: 2
X: 2

6 T: 0 6 2: 28
Z: 28 1: 2
Y: 2
X: 6

+ T: 0 N/A 2: 28
Z: 0 1: 8
Y: 28
X: 8

4 T: 0 4 2: 28
Z: 28 1: 8
Y: 8
X: 4

ENTER T: 28 N/A 3: 28
Z: 8 2: 8
Y: 4 1: 4
X: 4

6 T: 28 6 3: 28
Z: 8 2: 8
Y: 4 1: 4
X: 6

+ T: 28 N/A 3: 28
Z: 28 2: 8
Y: 8 1: 10
X: 10

X T: 28 N/A 2: 28
Z: 28 1: 80
Y: 28
X: 80

+ T: 28 N/A 1: 108
Z: 28
Y: 28
X: 108

2 T: 28 2 1: 108
Z: 28
Y: 108
X: 2

ENTER T: 28 N/A 2: 108
Z: 108 1: 2
Y: 2
X: 2

3 T: 28 3 2: 108
Z: 108 1: 2
Y: 2
X: 3

+ T: 28 N/A 2: 108
Z: 28 1: 5
Y: 108
X: 5

2 T: 28 2 2: 108
Z: 108 1: 5
Y: 5
X: 2

ENTER T: 108 N/A 3: 108
Z: 5 2: 5
Y: 2 1: 2
X: 2

(28 was overflowed, but it's no longer needed)

1 T: 108 1 3: 108
Z: 5 2: 5
Y: 2 1: 2
X: 1

+ T: 108 N/A 3: 108
Z: 108 2: 5
Y: 5 1: 3
X: 3

X T: 108 N/A 2: 108
Z: 108 1: 15
Y: 108
X: 15

3 T: 108 3 2: 108
Z: 108 1: 15
Y: 15
X: 3

ENTER T: 108 N/A 3: 108
Z: 15 2: 15
Y: 3 1: 3
X: 3

5 T: 108 5 3: 108
Z: 15 2: 15
Y: 3 1: 3
X: 5

+ T: 108 N/A 3: 108
Z: 108 2: 15
Y: 15 1: 8
X: 8

4 T: 108 4 3: 108
Z: 15 2: 15
Y: 8 1: 8
X: 4

ENTER Overflow!: 108 N/A 4: 108
T: 15 3: 15
Z: 8 2: 8
Y: 4 1: 4
X: 4

2 Overflow!: 108 2 4: 108
T: 15 3: 15
Z: 8 2: 8
Y: 4 1: 4
X: 2

+ Overflow!: 108 N/A 4: 108
T: 15 (Would be 108) 3: 15
Z: 15 2: 8
Y: 8 1: 6
X: 6

X Overflow!: 108 N/A 3: 108
T: 15 (Would be 108) 2: 15
Z: 15 (Would be 108) 1: 48
Y: 15
X: 48

+ Overflow!: 108 N/A 2: 108
T: 15 (Would be 108) 1: 63
Z: 15 (Would be 108)
Y: 15 (Would be 108)
X: 63

/ Overflow!: 108 N/A 1: '12/7' (Exact) or 1.71418571429 (Approximate)
T: 15 (Would be 108)
Z: 15 (Would be 108)
Y: 15 (Would be 108)
X: 0.238095238 (Would be 1.714285714)

So, if I've got that right, in Classis RPN, that last ENTER to
allow keying in the final 2 pushed the 108 into the bit bucket,
but with a 2-line display, that wouldn't be apparent until the
last + operation, and with a 1-line display, not until the very
last operation, / (and perhaps non-obvious to the unwary).

In RPL, it looks as if the most that was used was 4 stack levels
plus the command line, but of course the ENTER operations were
actually using the stack "behind the scenes", without the display
being updated until ENTER was finished.