I have encountered a double 'LockDisplay' situation inside XkbRefreshKeyboardMapping.  It calls LockDisplay, then XkbGetMapChanges:

        ...
        LockDisplay(dpy);
        if ((rtrn=XkbGetMapChanges(dpy,xkbi->desc,&changes))!=Success) {
        ...

Then XKBGetMapChanges *also* calls LockDisplay:

Status
XkbGetMapChanges(Display *dpy,XkbDescPtr xkb,XkbMapChangesPtr changes)
{
    xkbGetMapReq *req;

    if ((dpy->flags & XlibDisplayNoXkb) ||
        (!dpy->xkb_info && !XkbUseExtension(dpy,NULL,NULL)))
        return BadAccess;
    LockDisplay(dpy);
    ...

This only really is a problem when using xcb, due to the more strict  locking assertions.

The actual situation which led me to this discovery occurred in the 'synergys' application (http://synergy2.sourceforge.net/) which gave me the following traceback:

Locking assertion failure.  Backtrace:
#0 /usr/lib/libxcb-xlib.so.0 [0x2aff31e949ec]
#1 /usr/lib/libxcb-xlib.so.0(xcb_xlib_lock+0x15) [0x2aff31e94a77]
#2 /usr/lib/libX11.so.6 [0x2aff30ee8e32]
#3 /usr/lib/libX11.so.6(XkbGetMapChanges+0x3e) [0x2aff30f2bb08]
#4 /usr/lib/libX11.so.6(XkbRefreshKeyboardMapping+0xc7) [0x2aff30f28b4d]
#5 synergys [0x42f4a0]
#6 synergys [0x4311c8]
#7 synergys [0x457ac9]
#8 synergys [0x409206]
#9 synergys [0x4094ab]
#10 synergys [0x409ced]
#11 /lib/libc.so.6(__libc_start_main+0xf4) [0x2aff319681f4]
#12 synergys(__gxx_personality_v0+0x1e9) [0x4063c9]
synergys: xcb_xlib.c:73: xcb_xlib_lock: Assertion `!c->xlib.lock' failed.
Aborted