There are several reasons why mistakes occur when subtraction requires exchanging. The most common is due to treating subtraction as if it were commutative and switching the digits around instead of exchanging. This is particularly common when zero is in the top line of the calculation.
Once pupils understand the need to exchange, the exchanging may be incorrectly carried out.
Here, the exchanged ten has been added to the 3 as if it were a unit. This means that exchanging has to be repeated, with a further 1 being added to the units, before the subtraction is possible.
When such mistakes are made, using base 10 equipment to physically move the ten and see the result of doing so, will reinforce the understanding of how place value is retained in the written calculation.
See our How To video for an example of this.