This is not a scripting issue. It's an example of what I call "edit" locking (as opposed to using access controls to "lock" a record). If one user or a script they perform edits values in a record, other users cannot modify the values in that record at the same time. Replcace Field Contents, which need not be used in a script to encounter this issue, is just one such script step that can encounter this issue. Any other steps such as the "insert" steps and Set Field, will also edit lock records or be locked out of them by other uses editing the record.
For other issues, including the behavior differences for global fields in a networked database, see this knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/2984/kw/global%20fields%20network