This may seem like common sense, but you don't know how often I see the lack of that in many children, including my very careless boy.
His '6' can look like a '0'. His '5' sometimes looks like '6'. His teacher who wasn't too forgiving had decided to literally teach him a lesson so many times.
Other common handwriting mistakes are the cursive 'l' to be used to represent the unit of measurement for 'litre' and the small, superscript 'o' used to mean 'degree'.
One thing I tell my son to encourage him to tidy-up his handwriting is that neat and legible handwriting allows him to check his workings much quicker.
Experiment #7 - Forces (Frictional Force)
Experiment #6 - Forces (Frictional Force)