diff --git a/Annex/Content.hs b/Annex/Content.hs
index b9163ae44e..ad0b220381 100644
--- a/Annex/Content.hs
+++ b/Annex/Content.hs
@@ -12,6 +12,7 @@ module Annex.Content (
 	inAnnex',
 	inAnnexSafe,
 	inAnnexCheck,
+	objectFileExists,
 	lockContentShared,
 	lockContentForRemoval,
 	ContentRemovalLock,
@@ -131,6 +132,11 @@ inAnnex' isgood bad check key = withObjectLoc key checkindirect checkdirect
 				)
 			else checkdirect locs
 
+{- Like inAnnex, checks if the object file for a key exists,
+ - but there are no guarantees it has the right content. -}
+objectFileExists :: Key -> Annex Bool
+objectFileExists key = calcRepo (gitAnnexLocation key) >>= liftIO . doesFileExist
+
 {- A safer check; the key's content must not only be present, but
  - is not in the process of being removed. -}
 inAnnexSafe :: Key -> Annex (Maybe Bool)
diff --git a/CHANGELOG b/CHANGELOG
index 2229ceff36..11164036e5 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -12,6 +12,9 @@ git-annex (7.20181122) UNRELEASED; urgency=medium
   * info: When used with an exporttree remote, includes an "exportedtree"
     info, which is the tree last exported to the remote. During an export
     conflict, multiple values will be listed.
+  * dropunused: When an unused object file has gotten modified, eg due to
+    annex.thin being set, don't silently skip it, but display a warning
+    and let --force drop it.
 
  -- Joey Hess <id@joeyh.name>  Tue, 27 Nov 2018 12:29:27 -0400
 
diff --git a/Command/DropUnused.hs b/Command/DropUnused.hs
index c5a61d7391..d5394c2ea8 100644
--- a/Command/DropUnused.hs
+++ b/Command/DropUnused.hs
@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2010,2012 Joey Hess <id@joeyh.name>
+ - Copyright 2010,2012,2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -8,6 +8,7 @@
 module Command.DropUnused where
 
 import Command
+import qualified Annex
 import qualified Command.Drop
 import qualified Remote
 import qualified Git
@@ -48,9 +49,19 @@ perform from numcopies key = case from of
 		showAction $ "from " ++ Remote.name r
 		Command.Drop.performRemote key (AssociatedFile Nothing) numcopies r
 	Nothing -> ifM (inAnnex key)
-		( Command.Drop.performLocal key (AssociatedFile Nothing) numcopies []
-		, next (return True)
+		( droplocal
+		, ifM (objectFileExists key)
+			( ifM (Annex.getState Annex.force)
+				( droplocal
+				, do
+					warning "Annexed object has been modified and dropping it would probably lose the only copy. Run this command with --force if you want to drop it anyway."
+					next $ return False
+				)
+			, next $ return True
+			)
 		)
+  where
+	droplocal = Command.Drop.performLocal key (AssociatedFile Nothing) numcopies []
 
 performOther :: (Key -> Git.Repo -> FilePath) -> Key -> CommandPerform
 performOther filespec key = do
diff --git a/doc/bugs/dropunused_does_nothing/comment_3_8f80959997f6963728a8aee3765dc349._comment b/doc/bugs/dropunused_does_nothing/comment_3_8f80959997f6963728a8aee3765dc349._comment
new file mode 100644
index 0000000000..2206573e7e
--- /dev/null
+++ b/doc/bugs/dropunused_does_nothing/comment_3_8f80959997f6963728a8aee3765dc349._comment
@@ -0,0 +1,14 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 3"""
+ date="2018-12-04T15:46:05Z"
+ content="""
+Indeed, the object seems to be there, but it looks like `dropunused`
+probably for some reason fails its `inAnnex` check and so skips it.
+
+Does `git config annex.thin` output true? If so, and if the object file you
+found does not checksum to the right value, `dropunused` would skip it.
+
+That seems to me to be a bug, it probably should delete even modified files
+in this case. But I don't know if it's the bug you're seeing.
+"""]]
diff --git a/doc/bugs/dropunused_does_nothing/comment_4_7e04e71d0eb03185148ab544bc24724c._comment b/doc/bugs/dropunused_does_nothing/comment_4_7e04e71d0eb03185148ab544bc24724c._comment
new file mode 100644
index 0000000000..92ca25affa
--- /dev/null
+++ b/doc/bugs/dropunused_does_nothing/comment_4_7e04e71d0eb03185148ab544bc24724c._comment
@@ -0,0 +1,26 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 4"""
+ date="2018-12-04T15:52:52Z"
+ content="""
+Test case for the annex.thin with modified file bug:
+
+	git annex init
+	git annex upgrade
+	git config annex.thin true
+	touch foo
+	git add foo
+	git commit -m add
+	echo foo >> foo
+	rm foo
+	git commit -m rm -a
+	git annex unused
+	git annex dropunused 1
+	git annex unused
+
+Now, dropunused is supposed to honor numcopies, and if an object file
+has been modified, that's probably the only existing copy of that object,
+and so dropunused should refuse to drop it by default. There ought to be a
+warning, and the user should be able to use --force to override and drop it
+anyway. I've implemented that now.
+"""]]