When a student sees this error, it usually means there is a hidden blank space in the answer box where they might have clicked on the arrow up to enter an exponent but then used the backspace on their keyboard which doesn't clear the space being held for the exponent.
This error can be reproduced by entering something like a^b and then hitting the down exponent to leave the exponent. You can then arrow backwards to get up into the exponent, just to the right of the b, then hit backspace. If you submit the answer at that point, it will give you this error.
The good news is that it doesn't steal a submission or apply a deduction. The student simply needs to use the "Clear" button on the pallet to clear the invisible space and re-enter their answer.