When viewing messages with sorting order set to 'thread', pressing moving the cursor to the previous message causes a strange rendering problem if the locale use the UTF-8 encoding. I'll attach a screen shot.
Created attachment 59505 [details] xterm screenshot
Package versions, for reference: mutt-1.4-2 ncurses-5.2-27 slang-1.4.5-3 XFree86-4.2.0-50.10
Fixed with slang-1.4.5-5.