I have a ThinkPad T460s (Skylake). After undocking from an UltraDock with DP MST hub), docking one more time causes a failed assert; 

Please let me know if I should enable further debug options. 

[   245.560] (EE) 
[   245.560] (EE) Backtrace:
[   245.560] (EE) 0: /usr/bin/X (xorg_backtrace+0x56) [0x58ade6]
[   245.560] (EE) 1: /usr/lib64/xorg/modules/drivers/intel_drv.so (0x7f6f56438000+0x9c012) [0x7f6f564d4012]
[   245.560] (EE) 2: /usr/lib64/xorg/modules/drivers/intel_drv.so (0x7f6f56438000+0xa0e2b) [0x7f6f564d8e2b]
[   245.560] (EE) 3: /usr/lib64/xorg/modules/drivers/intel_drv.so (0x7f6f56438000+0xaa4ad) [0x7f6f564e24ad]
[   245.561] (EE) 4: /usr/bin/X (xf86Wakeup+0x1aa) [0x4764ca]
[   245.561] (EE) 5: /usr/bin/X (WakeupHandler+0x6b) [0x43a36b]
[   245.561] (EE) 6: /usr/bin/X (WaitForSomething+0x1dd) [0x587c5d]
[   245.561] (EE) 7: /usr/bin/X (0x400000+0x3557e) [0x43557e]
[   245.561] (EE) 8: /usr/bin/X (0x400000+0x397b3) [0x4397b3]
[   245.561] (EE) 9: /lib64/libc.so.6 (__libc_start_main+0xf0) [0x7f6f5a6a4790]
[   245.561] (EE) 10: /usr/bin/X (_start+0x29) [0x423aa9]
[   245.561] (EE) 
[   245.561] (EE) 
Fatal server error:
[   245.561] (EE) sna_output_add:5098 assertion 'to_sna_output(output)->id == 0' failed
[   245.561] (EE) 
[   245.562] (EE) 
Please consult the The X.Org Foundation support 
         at http://wiki.x.org
 for help. 
[   245.562] (EE) Please also check the log file at "/var/log/Xorg.0.log" for additional information.
[   245.562] (EE) 
[   245.562] (II) AIGLX: Suspending AIGLX clients for VT switch
[   246.282] (EE) Server terminated with error (1). Closing log file.

It was a bad assert - a timing issue where we might not see the disconnect before the reconnect.