Typing an example from the "Read Syntax for Circular Objects" section of
the manual into IELM can cause Emacs to hang or crash. I can reproduce
this fairly consistently with an Emacs built from master on my machine. It
took me several tries, but I was also able to reproduce it with the Emacs
25.1 installed by homebrew. Here are the steps:

emacs -Q
M-x ielm RET
#1=(a #1#) RET
C-p C-p RET

Result: (on OS X) rainbow spinning mouse cursor, unresponsive Emacs

There may be something timing-related going on, because these steps
don't work every time and they seem to work more consistently if I enter
the sequence of keystrokes quickly. If it doesn't happen the first time,
navigating again to a previous line and editing it by adding a quote
somewhere or another symbol and pressing RET again may cause it to
happen.

When I encountered this bug, I was working on one of my own branches
based on commit eb610f2 in master, and Emacs crashed and gave me a stack
trace, which I've attached. Since then I have not been able to reproduce
the crash, just the hang.