===========================================================

CASE 1:

Type the following equations into the calc:
A+B=C (eq1)
D+E=C (eq2)

Press [ON] to exit EQN mode
Press [ON]+[C] to soft-resetting calculator (try without it - I used, for safety)
Press [CLVARS] to clear variables
Go to EQN list, and search (eq1)
Press [SOLVE] C
On A?0 prompt type 2 [R/S]
On B?0 prompt press [ON] to cancel SOLVE
Go to EQN list, and search (eq2)
Press [ENTER]
On D?0 prompt type 3 [R/S]
On E?0 prompt type 7 [R/S]

The EQN anunciator will flashing and on the screen seems
C?0 or C?1, and you will get C=10.

===========================================================

CASE 2/a:

Type the following equations into the calc:
A+B=C (eq1)
D+E=F (eq2) <-- CHANGE IS HERE!!!

Press [ON] to exit EQN mode
Press [ON]+[C] to soft-resetting calculator (try without it - I used, for safety)
Press [CLVARS] to clear variables
Go to EQN list, and search (eq1)
Press [SOLVE] C
On A?0 prompt type 2 [R/S]
On B?0 prompt press [ON] to cancel SOLVE
Go to EQN list, and search (eq2)
Press [ENTER]
On D?0 prompt type 3 [R/S]
On E?0 prompt type 7 [R/S]
On F?0 prompt type 10 [R/S]

You will get C=1 or C=0.
(If you after CLVARS deleted the stack, you will get always C=1.)

===========================================================

CASE 2/b:

Type the following equations into the calc:
A+B=C (eq1)
D+E=F (eq2)

Press [ON] to exit EQN mode
Press [ON]+[C] to soft-resetting calculator (try without it - I used, for safety)
Press [CLVARS] to clear variables
Go to EQN list, and search (eq1)
Press [SOLVE] C
On A?0 prompt type 2 [R/S]
On B?0 prompt press [ON] to cancel SOLVE
Go to EQN list, and search (eq2)
Press [ENTER]
On D?0 prompt type 3 [R/S]
On E?0 prompt type 7 [R/S]
On F?0 prompt type 11 [R/S] <-- CHANGE IS HERE!!!

The EQN anunciator will flashing, and you will get NO ROOT FND error.

===========================================================

For a good reproduction try this with empty stack
(clear the stack after CLVARS).