This behaviour has been driving me batty.

Firstly, it makes it too easy to accidentally edit endpoints. e.g. http://beta.musicbrainz.org/edit/relationship/edit?returnto=http%3A%2F%2Fbeta.musicbrainz.org%2Frecording%2F12b182f3-5483-47d9-996e-41498b3b2fde&type1=url&type0=recording&id=1808

tab into the first field
press down
tab into the next field

result: It selects the first result from the lookup. There are no visible changes, so you have no way of knowing that it's changed and are quite likely to submit the edit without realising, as happened to me in http://beta.musicbrainz.org/edit/17174715 and several other edits I made today.

It also causes me a lot of problems and frustration when trying to fix artists in the release editor and the new artist doesn't yet exist.
e.g. On http://beta.musicbrainz.org/release/924b924e-edbf-462b-be29-12cca2f40551/edit try and change SI☆NA on track 18 to a currently non-existing artist called "si-na".

• First you write "si-na" and see that there are no matching artists.
• The artist doesn't exist so the only option to unlink the current artist is to select "Clear artist", which helpfully also deletes the text you entered.
• Then you write "si-na" again and tab out of the field without selecting anything...
  result: It selects SI☆NA again

I've given up trying to fix a number of releases (including that one) now.