This problem was reported last month and then again earlier this month, for 
example here:

https://lists.gnu.org/archive/html/emacs-orgmode/2014-10/msg00178.html

I'm running Emacs 25.0.50.2, Org 8.2.10 and ESS 14.1x.  I'm getting a lot of

    Error: could not find function ".ess.eval"

errors when evaluating R code blocks, even simple ones like this:

   #+BEGIN_SRC R :session :exports results
   1 + 1
   #+END_SRC

I have a more complicated Org file with a bunch of R in it that I want to export 
to a PDF, but I get this:

    Args out of range: #<killed buffer>, 0 ,1

I've been trying to debug it and seemed to narrow down a minimal example to 
whether or not a simple block had ":exports results" in it, but yesterday when 
working on this some process would go haywire and my machine would slow down 
and then freeze!  I had to force a reboot.  I've never had Emacs do that before.

I see that a patch was introduced for ob-R.el that fixes the 'could not find 
function ".ess.eval"' problem---will that be pushed into the main release soon?

And has anyone else been running into problems like this recently?  From what I 
saw in the archives it looks like it went away for most people.

Bill
-- 
William Denton ↔  Toronto, Canada ↔  https://www.miskatonic.org/