This bug occurs in Tremulous when playing the arachnid2 map. I don't know how to trigger this bug, I just move around the level randomly until it occurs.

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7f32879f6720 (LWP 26668)]
0x000000000049f9c7 in R_FogFactor (s=nan(0x7fffff), t=nan(0x7fffff)) at src/renderer/tr_image.c:1024
1024		d = tr.fogTable[ (int)(s * (FOG_TABLE_SIZE-1)) ];
(gdb) print s
$1 = nan(0x7fffff)
(gdb) bt
#0  0x000000000049f9c7 in R_FogFactor (s=nan(0x7fffff), t=nan(0x7fffff)) at src/renderer/tr_image.c:1024
#1  0x00000000004b758c in RB_CalcModulateAlphasByFog (colors=0x1115060 "��\231���\231���\231���\231�", '�' <repeats 124 times>) at src/renderer/tr_shade_calc.c:765
#2  0x00000000004b4414 in ComputeColors (pStage=0x7f327e9107c0) at src/renderer/tr_shade.c:933
#3  0x00000000004b4b4b in RB_IterateStagesGeneric (input=0x11017e0) at src/renderer/tr_shade.c:1061
#4  0x00000000004b4e36 in RB_StageIteratorGeneric () at src/renderer/tr_shade.c:1188
#5  0x00000000004b53cd in RB_EndSurface () at src/renderer/tr_shade.c:1447
#6  0x000000000049005d in RB_RenderDrawSurfList (drawSurfs=0x7f327ae80020, numDrawSurfs=690) at src/renderer/tr_backend.c:565
#7  0x0000000000490ebd in RB_DrawSurfs (data=0x7f327afb04f8) at src/renderer/tr_backend.c:904
#8  0x0000000000491363 in RB_ExecuteRenderCommands (data=0x7f327afb04f8) at src/renderer/tr_backend.c:1074
#9  0x0000000000498566 in R_IssueRenderCommands (runPerformanceCounters=qtrue) at src/renderer/tr_cmds.c:157
#10 0x0000000000498b19 in RE_EndFrame (frontEndMsec=0x0, backEndMsec=0x0) at src/renderer/tr_cmds.c:433
#11 0x000000000041d28d in SCR_UpdateScreen () at src/client/cl_scrn.c:508
#12 0x0000000000417f7a in CL_Frame (msec=12) at src/client/cl_main.c:2278
#13 0x000000000043b792 in Com_Frame () at src/qcommon/common.c:2746
#14 0x00000000004c9126 in main (argc=7, argv=0x7fff8fbe4028) at src/sys/sys_main.c:607