This jira describes two defects, and one usability improvement:

DEFECT 1:
When assigning a jira by the name of the assignee, you usually start typing the name of the assignee, and then select the assignee from the filtered list.

When a user needs to re-assign a huge number of jiras to the same person, it is cumbersome to type the name over and over. Its easier to use the filtered list the first time, then copy the full name as provided in the list, and paste it into the assignee field for all other jiras you want to assign to that same person.

The issue is that when you paste that full name into an assignee field, you are forced to re-select that person from the auto-complete field (which results in the same name being set anyway). If you remove focus from the assignee field after pasting in the full name, and without selecting the user from the filtered list, then the assignment fails with: "User '??????' does not exist." - even through it is the exact name of that user.

This happens in both the Assign Dialog, and the Assignee editable field (under people) on the jira page, however the error in the Assign dialog doesn't appear until you click the Assign button on the dialog.

Exact steps:
Locate any editable existing jira, which is currently assigned to a user (lets say "Bob Smith").
Edit the assignee field, and copy the exact full name (Bob Smith).
Delete the text in the assignee field.
Paste in the copied text (Bob Smith).
The filter will come up with the match for Bob Smith. Don't select this - just click outside of the text field (shift focus) to confirm your edit of Bob Smith.
Red error message is displayed underneath saying "User 'Bob Smith' does not exist."

I'm guessing that the actual assignee being set in the background is not the name of the user, but his email address or user Id, and to prevent accidental incorrect assignment in the case of two users having the exact same full name... however even in large organisations that is not a very likely occurrence. For usability sake, if the user makes an edit to the assignee field and the field loses focus, you could do and ajax check for an exact, distinct match on the supplied name, and if so - assign that user Id/email transparently in the background, otherwise (if no match or non-distinct match) display the usual error.
Once you remove focus from the field, there is an ajax request and a busy-spinner is displayed while waiting - in that request you could supply the entered text, and look for a distinct match on name...

DEFECT 2:
In the error scenario above, if you have pasted in the exact full name, shifted focus, and received the error message "User 'Bob Smith' does not exist.", and you attempt to fix it by opening the dropdown box on the assignee field (which is visible while the field is in edit mode) to reselect the user from the list; then the list opens, but filtering fails, and displays "No Matches", even though you have the exact full name of that user in the text edit field.
You are forced to once again delete the name form the field, and type it in again.

This happens in both the Assign Dialog, and the Assignee editable field (under people) on the jira page, however the error in the Assign dialog the "No Matched" on dropdown-click doesn't appear until after you've tried to click the Assign button on the dialog, and received the error message.

USABILITY:
While assigning jiras and typing in the assignee name, the list of filtered users with a partial match of the entered text is displayed, however this filtered list is very slow (we use an Atlassian hosted Jira). Surely you could keep a list of most used users in a cookie / local storage in the browser, and use that as fast filtering then assigning?

Also, if you don't use the filtered list, but just click the dropdown, a list of "Suggestions" are displayed. This list is not populated very well. The users I assign jiras to most ofter are not in this list... instead its got names in of users I've not interacted with for a very long time, or very infrequently. This list of suggested users should be populated by users you most frequently interact with. Possibly filtered by Jira project.

This is noted on our Atlassian hosted Jira: Atlassian JIRA (v6.1.7#6163-sha1:94d557d), but might affect newer versions also.