Editing user fields in PHP locks user out

I have recently written a custom module that among other things edits a
user field (2 actually) during execution. If any user field is changed,
after logging out, the user can no longer log in and has to reset his/her
password. It this usual behaviour (am I missing something key to drupal
permissions?)

I can send code, but it is all using user_load ---- modify array -- and
then user_set(array) to save the changes

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