I know this may be a pathological case, but I found it while debugging PopCon: Invoking a debugged function prior to the conclusion of setup_Rmainloop() will cause R to crash because R_ConsoleIob is not initialized until run_Rmainloop(). Tested in R-2-6-branch and trunk on Linux. Crash it like this from a shell command line: $ cat >> ~/.Rprofile <<EOF foo <- function() x<-1 debug(foo) foo() EOF $ $R R version 2.7.0 Under development (unstable) (2008-02-15 r44481) Copyright (C) 2008 The R Foundation for Statistical Computing ISBN 3-900051-07-0 R is free software and comes with ABSOLUTELY NO WARRANTY. You are welcome to redistribute it under certain conditions. Type 'license()' or 'licence()' for distribution details. Natural language support but running in an English locale R is a collaborative project with many contributors. Type 'contributors()' for more information and 'citation()' on how to cite R or R packages in publications. Type 'demo()' for some demos, 'help()' for on-line help, or 'help.start()' for an HTML browser interface to help. Type 'q()' to quit R. debugging in: foo() debug: x <- 1 Browse[1]> q *** caught segfault *** address (nil), cause 'memory not mapped' Traceback: 1: foo() Possible actions: 1: abort (with core dump, if enabled) 2: normal R exit 3: exit R without saving workspace 4: exit R saving workspace Selection: Jeff -- http://biostat.mc.vanderbilt.edu/JeffreyHorner