diff --git a/src/internfile/mimehandler.cpp b/src/internfile/mimehandler.cpp
index 0557524b..9b8ae4ee 100644
--- a/src/internfile/mimehandler.cpp
+++ b/src/internfile/mimehandler.cpp
@@ -42,6 +42,11 @@ using namespace std;
 // handlers. There can be several instances for a given mime type
 // (think email attachment in email message: 2 rfc822 handlers are
 // needed simulteanously)
+//
+// FIXME: this is not compatible with multiple threads and a potential
+// problem in the recoll gui (indexing thread + preview request). A
+// simple lock would be enough as handlers are removed from the cache
+// while in use and multiple copies are allowed
 static multimap<string, Dijon::Filter*>  o_handlers;
 
 /** For mime types set as "internal" in mimeconf: