I finally caught an example, which was not immediately after restart.  I copied the two lines of output, entered 'import re\n' and the copied text was pasted, then the line with the addition compiled, leading to the error.

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>>> for c in '℘·': hex(ord(c))
... 
'0x2118'
'0xb7'
>>> import re'0x2118'
... '0xb7'
SyntaxError: invalid syntax
>>> for c in '℘·': hex(ord(c))

I pasted the copied output into an issue on the browser and did other stuff before coming back to this window to try something with re.

Differences from my initial report: No RESTART (though usually or often is), no exception, not first entry (but first time not?).  

Edit menu item 'Paste' and the shortcut are bound to IDLE-defined pseudoevent '<<paste>>' which invokes method EditorWindow.paste which calls event_generate('<<Paste>>'), where the latter event is a tk-defined event. Where triggered?  I could add a print-to-console in .paste?  Happening only in shell and after Enter suggest looking as shell-specific part of Enter handling.  If never on Linux, could x-11-only fix-x11-paste (called in pyshell.main) explain why not?

I searched stackoverflow [tkinter] questions for 'paste'.  Got 840 responses, looked as first page, saw nothing relevant.  Adding 'unwanted', 'unexpected', or 'spontaneous' resulted in no hits.  I will consider asking a question myself.