Example case:

2 node replica set:
primary tag:

{ dc: 'sf' }

secondary tag:

{ dc: 'ny' }

read pref setting:
mode: secondary
tags: [

{ dc: 'ny' }

]

If the secondary was down and the went up again, the replica set connection object will attempt to select nodes again (since the first try gave us nothing due to the node being down and marked as bad). However, the iterator for the tag was already moved on the first attempt and the retry would end up not checking the nodes because it already depleted the tags on the first attempt.