Please include patch or file to commit.
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Add field_time_entries key for both zh-TW and en locales.

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Maybe this string from some plugin? In core distribution this string not exist.
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I can reproduce this issue (I'm using Redmine from the trunk in revision 3735):

# create a required custom field for spent time,
# create new time entry without value in the field.

One of the two error messages, which appear as a result of these steps, is the message reported in this ticket.
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I can also reproduce this. It only occurs, if you try to create a TimeEntry during an issue update while that particular TimeEntry does not validate.

This is probably cause by a change to the behavior during issue update. The cause is probably in Issue#save_issue_with_child_records.
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Even i can reproduce this issue.
This issue appears to be very obvious.
I don't know how it missed your developers eyes.

Steps to reproduce
1) Create an issue and fill all required fields
2) Update the issue by entering a perfectly valid value in "Spent time" field, but don't select any options in "Activity" drop down list.

The issue appears to be not in validating the value entered in "Spent time" field, as 3, 7 are all perfectly valid values for Hour based field.

More the issue also not appeared to be related to Translation.
As i have using US English version, for that it self i am getting the following error.

"translation missing: en,
field_time_entries is invalid"
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Moreover Using custom field for "Spent time" is totally unnecessary to reproduce this issue.
So this issue is nothing to do with Custom Field.
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My guess is the "extra" validation error happens here source:/trunk/app/models/issue.rb#L495 . The problem appears to happen when you try to add an invalid time entry while updating the issue, this _does not_ happen when logging time "on its own" through the "Log time" link. The flash then shows the validation errors for the TimeEntry as well as an error that the TimeEntry is invalid (probably as part of the error messages from the issue).
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OK, new string added in r4007.
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duplicates,Closed,6055,Missing Translatione error when logging time without Activity