Odd error message when editing field settings (Variable "targetLocaleFieldHtml" does not exist)
This had me stumped for a while, but it turned out to be a custom fieldtype extending BaseElementFieldType, in which the settings attribute targetLocaleField apparently was renamed targetLocaleFieldHtml with Craft 2.5.2750.

Not sure why the error was thrown for all fields and fieldtypes (even PlainText fields), but in any case the fix was simply replacing the targetLocaleField attribute with targetLocaleFieldHtml in the returned array for the getSettingsHtml() method in the custom fieldtype.

Edit: It just occurred to me that the reason for getting the error on all fields/fieldtypes is – of course – due to Craft actually pre-rendering the settings templates for all fieldtypes, I assume to make changing the fieldtype (via the dropdown) seamless/instantaneous.