Description
The steps:
Step 1.
Add an object {_id : 1, a : [1, 2]} to the empty collection test:
> db.test.drop()
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> db.test.insert({_id : 1, a : [1, 2]})
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Step 2.
Let's try to pop from a[-2], i.e. from non existent data. This is nonsense perhaps but it changed behaviour.
> db.test.update({_id : 1}, {$pop : {"a.-2" : 1}})
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WriteResult({
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"nMatched" : 0,
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"nUpserted" : 0,
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"nModified" : 0,
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"writeError" : {
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"code" : 16837,
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"errmsg" : "cannot use the part (a of a.-2) to traverse the element ({a: [ 1.0, 2.0 ]})"
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}
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})
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The above pop fails and data are not changed. In v2.4.9 this nonsense pop was silently ignored (if I am not mistaken).
Step 3.
Now let's pop from a[3], i.e. also from non existent data.
> db.test.update({_id : 1}, {$pop : {"a.3" : 1}})
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WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
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> db.test.find()
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{ "_id" : 1, "a" : [ 1 ] }
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The result is presumably incorrect. {$pop : {"a.3" : 1}} instead of failing (like with a.-2 in v2.6) or doing nothing (like with the same case in v2.4.9) it removes data from 'a' itself, i.e. actually performs another operation: {$pop : {"a" : 1}}.