I hope "critical" is the right choice of severity. Function "stem" is in the "graphics" package. Apologies if neither of these is correct.
Example of problem (R 3.0.1, Platform: x86_64-apple-darwin10.8.0 (64-bit)) ...
> stem(c(1, Inf))
Control never returned. Have to kill R process. Can't CTRL-C
Compare to (R2.15.1, Platform: x86_64-redhat-linux-gnu (64-bit)) which works as expected...
Error in stem(c(1, Inf)) : NA/NaN/Inf in foreign function call (arg 1)