Using unity-2d from daily ppa version 4.8-0ubuntu1~bzr716 if I start with two screens, unplug one and plug it back in immediately after it switches to single screen but before the panel has been redrawn then sometimes I end up with no panel at all. dmesg shows the output below (this may be the result of several screen switches as it took a few goes to get it to fail). Let me know if there is anything else that may be useful. I do not know whether this is a new symptom or whether it is to do with recent upgrades.
[ 743.876856] show_signal_msg: 18 callbacks suppressed
[ 743.876863] unity-2d-panel[3544]: segfault at f845ffa5 ip 056a6f08 sp bfd649ac error 5 in libc-2.13.so[5638000+176000]
[ 828.417599] unity-2d-panel[4104]: segfault at d107c335 ip 00528f08 sp bfddeb3c error 5 in libc-2.13.so[4ba000+176000]
[ 886.935720] unity-2d-panel[4218]: segfault at 39f51415 ip 076bcf08 sp bf87ebbc error 4 in libc-2.13.so[764e000+176000]
[ 888.844492] [drm:drm_edid_block_valid] *ERROR* EDID checksum is invalid, remainder is 8
[ 888.844497] Raw EDID:
[ 888.844501] <3>00 ff ff ff ff ff ff 00 41 0c 11 c0 81 94 06 00 ........A.......
[ 888.844504] <3>26 0f 01 03 1f 22 1b 78 ee 1f f5 a4 57 47 9d 24 &....".x....WG.$
[ 888.844507] <3>13 50 54 bf ef 80 81 80 01 01 01 01 01 01 01 01 .PT.............
[ 888.844510] <3>01 01 01 01 01 01 30 2a 00 98 51 00 2a 40 30 70 ......0*..Q.*@0p
[ 888.844513] <3>13 00 52 0e 11 00 00 1e 00 00 00 ff 00 20 42 5a ..R.......... BZ
[ 888.844515] <3>20 20 34 33 31 32 3b 33 0a 20 00 00 00 fc 00 50 4312;3. .....P
[ 888.844518] <3>68 69 6c 69 70 73 20 31 37 30 43 0a 00 00 00 fd hilips 170C.....
[ 888.844521] <3>00 38 4c 1e 53 0e 00 0a 20 20 20 20 20 20 00 df .8L.S... ..
[ 888.844523]
[ 928.892683] unity-2d-panel[4552]: segfault at 894da1cd ip 00456f08 sp bfc4923c error 4 in libc-2.13.so[3e8000+176000]
[ 1002.704998] unity-2d-panel[4724]: segfault at 8996304d ip 07d53f08 sp bfe44eec error 4 in libc-2.13.so[7ce5000+176000]
[ 1028.542372] unity-2d-panel[4862]: segfault at d123be8f ip 006c4f17 sp bf9186ac error 5 in libc-2.13.so[656000+176000]
[ 1167.316188] unity-2d-panel[5055]: segfault at c8b8848d ip 00506f08 sp bfa738ac error 5 in libc-2.13.so[498000+176000]
[ 1193.381080] unity-2d-panel[5113]: segfault at e98ebcbd ip 06119f08 sp bfd10fac error 5 in libc-2.13.so[60ab000+176000]
[ 1323.593331] unity-2d-panel[5405]: segfault at 8038a1d5 ip 0041af08 sp bf8a8fac error 4 in libc-2.13.so[3ac000+176000]
[ 1359.473681] unity-2d-panel[5599]: segfault at ca1e97ad ip 06e22f08 sp bf86647c error 5 in libc-2.13.so[6db4000+176000]
Can't you install debug symbols, please?
However I'm wondering if this is a duplicate of bug #846836