Citation needed. I get that undefined behavior is hard to spot, but having spent years writing cross-platform C I cannot accept that every large program depends on it without some evidence.

I'm not finding an academic citation, but perhaps a respected academic's blog counts? I think John Regehr is the likely source of these claims. In his words:

My guess is that most or all executions of GCC
and LLVM (and basically all other large programs)
would be undefined.

http://blog.regehr.org/archives/761

He starts the post by stating that the "majority of the programs in SPEC CINT 2006 execute undefined behavior during the course of a 'reportable' run". A majority is definitely not "all", but the SPEC programs are also not "large":

Let’s consider the SPEC benchmark programs. Making them fast is something that most compiler vendors would like to do. Using IOC we found that a majority of the programs in SPEC CINT 2006 execute undefined behavior during the course of a “reportable” run.

For large programs, he then looks at the GCC and Clang compilers themselves, and concludes:

If we consider them to be written in C99, LLVM/Clang 3.1 and GCC (SVN head from July 14 2012), running on Linux on x86-64, do not appear to admit any non-trivial, non-erroneous executions. I expect the same is true of most or all previous versions of these compilers. IOC tells us that these compilers execute undefined behaviors even when compiling an empty C or C++ program with optimizations turned off.

Lest you think that the undefined behavior is caused by inappropriately forcing C99 on earlier code, he goes on to explain that the problems probably exist there too, but that the lack of tooling for earlier dialects prevents proper analysis:

My guess is that most or all executions of GCC and LLVM (and basically all other large programs) would be undefined by C89 (and not just C99) but we can’t tell for sure without using a comprehensive undefined behavior checker for C89. This tool does not exist. I confess to not personally having the gumption necessary for cramming GCC or LLVM through the best available dynamic undefined behavior checkers: KCC and Frama-C.

His overall message (which I think is true and merits significant reflection) is that C's utility as a real world language is based on unspoken agreements with compiler writers to use some degree of judgment and restraint when making optimizations based on undefined behavior. Rather than a standardized language defined by the written specifications, we are dependent on an "undocumented dialect of C where undefined behavior can sometimes be exploited and sometimes not":

This means that a correct C/C++ compiler could generate code that is almost infinitely faster than the code generated by current compilers. The SPEC harness would of course complain that the compiled programs produce the wrong results, but this is actually a bug in the SPEC harness, not a bug in the compilers. If we insist that a compiler turns these SPEC programs into executables that produce the specified results, then we are effectively mandating that this compiler implements a bizarre, undocumented dialect of C where undefined behavior can sometimes be exploited and sometimes not.

http://blog.regehr.org/archives/761