They may be overly focused on speed rather than quality.
This can tempt some people into rushing through issues to clear their list and see what comes back in bug reports later.
To rectify this balance:
- assign only a couple of items at a time in your issue tracking system,
- code review and test anything they have "completed" as soon as possible so it will be back with them immediately if there are any problems
- talk to them about your expectations about how long an item will take to do properly
