When comparing various files that reference the DontZap ServerFlags option, I've seen what appear to be contradictory indications about how the option is to be enabled/disabled.

In the file hw/xfree86/xorgconf.cpp, we have the following:

XCOMM Uncomment this to disable the <Ctrl><Alt><BS> server abort sequence
XCOMM This allows clients to receive this key event.

XCOMM    Option	"DontZap"	"false"

This was changed in revision 2b00afec4b5b51851aee4cb0fba7baff3e10ef3f such:

-XCOMM Uncomment this to enable the <Ctrl><Alt><BS> server abort sequence
-XCOMM The default allows clients to receive this key event.
+XCOMM Uncomment this to disable the <Ctrl><Alt><BS> server abort sequence
+XCOMM This allows clients to receive this key event.

XCOMM Option "DontZap" "false"

This indicates one should use the option "false" if one wants to disable the server abort sequence.

On the other hand, the man page hw/xfree86/doc/man/Xorg.man.pre says the following:

Ctrl+Alt+Backspace
Immediately kills the server -- no questions asked. It can be disabled by
setting the
  DontZap
__xconfigfile__(__filemansuffix__) file option to a TRUE value.

This was changed in revision 753310837cd3812882d8de67f063bb61813db675 such:

-Immediately kills the server -- no questions asked. This is disabled by
-default. It can be enabled with the -retro command line flag or by setting
-the
+Immediately kills the server -- no questions asked. It can be disabled by
+setting the
.B DontZap
-__xconfigfile__(__filemansuffix__) file option to a FALSE value.
+__xconfigfile__(__filemansuffix__) file option to a TRUE value.

This indicates one should use the option "true" if one wants to disable the server abort sequence.

Note as well the deletion of the reference to the "-retro" command line flag. While that one was removed, another equivalent reference in the file man/Xserver.man remains thus: "For the Xorg server, this also sets the default for the DontZap option to FALSE." Presumably if one reference was now redundant, so was the other... (I mention this in part because it only added to my confusion in trying to decipher the behaviour of the system I was trying to configure, which the upstream vendor had enabled -retro on by default.)

Finally, in the file hw/xfree86/doc/man/xorg.conf.man.pre, we have this:

.BI "Option \*qDontZap\*q  \*q" boolean \*q
This disallows the use of the
.B Terminate_Server
XKB action (usually on Ctrl+Alt+Backspace, depending on XKB options).
This action is normally used to terminate the __xservername__ server.
When this option is enabled, the action has no effect.
Default: off.

(They do all agree the default is that the terminate server action is enabled...)

I'm assuming that the file hw/xfree86/xorgconf.cpp is the one that's incorrect. (I haven't tested my assumption, as the machine in question isn't at hand right now.)