(mem (plus (mem (plus ...

[DJ Delorie]

> (insn 93 360 96 (set (mem/s:SI (plus:SI (mem:SI (plus:SI (reg/f:SI 29 $fp)

I hit this about a month ago.  In my case, I had defined
GO_IF_LEGITIMATE_ADDRESS in terms of a function, but neglected to pass
something indicating REG_OK_STRICT to that function, and wasn't
careful about not allowing pseudos during/after reload (i.e. referring
to other functions that didn't check REG_OK_STRICT either).  After I
cleaned up that function (including being careful about using other
macros which needed to be strict-sensitive), my problem went away.