During the past week I have started getting this error when performing
an update in Mongo. This was a working query and now it fails.

> db.mycollection.update(

{ "a.b" : 665 }

, { $pop : { "c" :

{ "d" : 1153 }

} });

"can't apply mod in place - shouldn't have gotten here"

mongo.log shows the following
"Tue Dec 7 16:10:12 [conn4] update liststore.individuals query:

{ a.b: 665.0 }

exception 10144 can't apply mod in place - shouldn't
have gotten here 0ms"

"a.b" is an indexed long
"c" is a collection containing an array
"d" is a field for each document within "c"

OS: Debian x64
Mongo: 1.6.4

Answer: Turns out that $pull, should be used instead of $pop.

The error message was not clear and could use a little attention