Program received signal SIGSEGV, Segmentation fault.
0x4002dfbc in mb_pixbuf_img_plot_pixel (pb=<value optimized out>, img=0x1ba40, x=22, 
    y=-434, r=255 'ÿ', g=0 '\0', b=0 '\0') at mbpixbuf.c:2087
(gdb) bt
#0  0x4002dfbc in mb_pixbuf_img_plot_pixel (pb=<value optimized out>, img=0x1ba40, x=22, 
    y=-434, r=255 'ÿ', g=0 '\0', b=0 '\0') at mbpixbuf.c:2087
#1  0x0000966c in paint_callback (app=<value optimized out>, drw=46137348)
    at mb-applet-system-monitor.c:272
#2  0x4002b8f6 in handle_expose (mb=0x1ba40, event=<value optimized out>) at mbtray.c:820
#3  0x4002b922 in mb_tray_app_repaint (mb=0x1ba40) at mbtray.c:833
#4  0x00008fd4 in timeout_callback (app=0x1ba40) at mb-applet-system-monitor.c:347
#5  0x4002c580 in mb_tray_app_main (mb=0x14008) at mbtray.c:302
#6  0x00009878 in main (argc=1, argv=0xbecf1ca4) at mb-applet-system-monitor.c:410
(gdb) up
(gdb) print membox_h
$1 = 20
(gdb) print mem_pixels
$2 = 18798069
(gdb) print memsize
$3 = 1597228901
(gdb) print msd
$4 = {loadIndex = 15, samples = 16, load = 0x18a90, total = 0x18b18, 
  mem_used = 18446744073706139648, mem_max = 63750144, swap_used = 0, swap_max = 0, 
  swap_percent = 0, mem_percent = 1597228901}

Huh, mem_used doesn't look good!

root@zaurus:~# printf %lx\\n -18446744073706139648
341000

Well, it's negative. The algorithm is a bit stange.

root@zaurus:~# cat /proc/meminfo 
MemTotal:        62256 kB
MemFree:          8504 kB
Buffers:          1508 kB
Cached:          24692 kB
SwapCached:       8604 kB
Active:          22632 kB
Inactive:        21344 kB
SwapTotal:      131064 kB
SwapFree:        72060 kB
Dirty:               0 kB
Writeback:           0 kB
AnonPages:       12636 kB
Mapped:           5532 kB
Slab:             4300 kB
SReclaimable:     1456 kB
SUnreclaim:       2844 kB
PageTables:       1116 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
WritebackTmp:        0 kB
CommitLimit:    162192 kB
Committed_AS:    81568 kB
VmallocTotal:   581632 kB
VmallocUsed:      9112 kB
VmallocChunk:   565244 kB

Now let's compute what's happening:

cache_used = cache_total - cache_free;
my_mem_used     = cache_used + used - cached - buffers;
12636 - 5532 + 8604 - 24692 - 1508 = -10492
				     ======

Well, I don't know, how to compute a really used memory, but I could at
least sanitize the value. Here you are:

Index: matchbox-panel-0.9.3/applets/mb-applet-system-monitor.c
===================================================================
--- matchbox-panel-0.9.3/applets/mb-applet-system-monitor.c	2006-02-07 22:19:12.000000000 +0000
+++ matchbox-panel-0.9.3/applets/mb-applet-system-monitor.c	2009-02-21 13:19:41.000000000 +0000
@@ -195,6 +195,11 @@
       my_mem_max      = total;
       my_swap_max     = cache_total;
       my_mem_used     = cache_used + used - cached - buffers;
+      /* Ugly hack! Fix mem-used algorithm is needed. */
+      if (my_mem_used < 0)
+	my_mem_used = 0;
+      if (my_mem_used > my_mem_max)
+	my_mem_used = my_mem_max;
       msd.mem_used    = my_mem_used;
       msd.mem_max     = my_mem_max;
       msd.mem_percent = (100 * msd.mem_used) / msd.mem_max;