Copying and pasting the degree symbol into the abstract field seems to cause the following error in IE, but not in FF.
Can't call method "fields" on an undefined value at
/home/koha/kohaclone/C4/Biblio.pm line 2101.
I couldn't reproduce this in IE8 on Windows 7. I added a degree symbol (Unicode character U+00B0, Windows keystroke Alt+0176) to 5xx fields and was able to save the record without error.
This could be an issue with Unicode normalization... NFC v. NFD...
I did the same as Owen and no problems saving. Maybe some NFC work done after 2012-06-29 has fixed it. I Used IE 8 on Win7.
Two attempts to reproduce this bug failed. I'm going to consider this fixed. Please re-open if you can provide more details about how to reproduce.