I regularly scan files in picard with no tags set, and expect the acoustids to match the file to the correct track. In some cases, it matches to the wrong track, despite the acoustid scores clearly denoting the correct track.

E.g. on a lookup which gives the result:
<response>
<status>ok</status>
<results>
<result>
<recordings>
<recording>
<title>夏色サプライズ (Instrumental)</title>
<id>0b62ef19-bbcf-4ee7-8008-4ea9de8eb163</id>
</recording>
</recordings>
<score>1.0</score>
<id>f31e17f9-5ef1-4947-b7ee-1f49ff343ac9</id>
</result>
<result>
<recordings>
<recording>
<releasegroups>...</releasegroups>
<title>夏色サプライズ</title>
<id>7351cdd5-e67a-4fdf-a969-697c1a2b7f7b</id>
</recording>
</recordings>
<score>0.466316</score>
<id>699f69c0-23fd-47ad-8723-32af76943e68</id>
</result>
</results>
</response>

(trimmed for clarity), picard will match to the recording which gets the lower score:

D: 140237869360896 01:41:21 'Looking up the fingerprint for file /home/cwalton/Downloads/Incoming/zomg/flac/04.flac...'
D: 140237869360896 01:41:21 POST-DATA 'clie ... (trimmed, result from this request above)
D: 140237869360896 01:41:21 'File /home/cwalton/Downloads/Incoming/zomg/flac/04.flac identified!'
D: 140237869360896 01:41:21 Updating file <File #6 u'04.flac'>
D: 140237869360896 01:41:21 Moving <File #6 u'04.flac'> from <Cluster u'Unmatched Files'> to <Cluster u'Unmatched Files'>
D: 140237869360896 01:41:21 Moving <File #6 u'04.flac'> from <Cluster u'Unmatched Files'> to <Track 7351cdd5-e67a-4fdf-a969-697c1a2b7f7b u'\u590f\u8272\u30b5\u30d7\u30e9\u30a4\u30ba'>

in this case, it should have picked 0b62ef19-bbcf-4ee7-8008-4ea9de8eb163 instead.