Further to my message below, I have reproduced the bug by loading the  
attached file, making the frame small and scrolling back and forth  
with the scrollbar. The two stack traces looks different, though.

The mode-line reads PDFLaTeX/F Ref BCite ll Noweb NWFL for this file.

The problem appears to be related to Noweb Mode, which seems to come  
with R-mode. It switches back and forth between LaTeX-mode (from  
AUCTeX), fundamental-mode and ESS-mode. (Problem is only reproducible  
is noweb-mode is on.)

Secondly, changing the major mode triggers a few things in `aquamacs- 
styles-mode': different faces and frame parameters are applied to the  
frame that displays a window with that buffer. But of course that  
shouldn't crash Emacs...