e.g.:

/Library/Fonts/BigCaslon.ttf : No FontName. Skipping.

and (more spectacularly):

/Library/Fonts/Chalkduster.ttfsh: line 1: 86587 Abort trap: 6
'/Library/Frameworks/R.framework/Versions/3.0/Resources/library/Rttf2pt1/exec//ttf2pt1' -a -GfAe '/Library/Fonts/Chalkduster.ttf' '/var/folders/jd/hrv5pvps34lflzkmt6q1czsc0000gp/T//RtmpAT3LLx/fonts/Chalkduster' 2>&1
 : No FontName. Skipping.

I also received a number of warnings (this is an excerpt):

38: In readLines(fd, 30) : seek on a gzfile connection returned an internal error
39: In readLines(fd, 30) : seek on a gzfile connection returned an internal error
40: In grepl("^FamilyName", text) : input string 4 is invalid in this locale
41: In grepl("^FontName", text) : input string 4 is invalid in this locale
42: In grepl("^FullName", text) : input string 4 is invalid in this locale
43: In grepl("^Weight", text) : input string 4 is invalid in this locale

Mac OS X 10.7.5, R 3.0.1
R version 3.0.1 (2013-05-16)
Platform: x86_64-apple-darwin10.8.0 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] splines stats graphics grDevices utils datasets methods base

other attached packages:
[1] extrafont_0.15 devtools_1.3 NADA_1.5-5 survival_2.37-4

loaded via a namespace (and not attached):
[1] colorspace_1.2-3 dichromat_2.0-0 digest_0.6.3 evaluate_0.4.7 extrafontdb_1.0
[6] ggplot2_0.9.3.1.99 grid_3.0.1 gtable_0.1.2 httr_0.2 labeling_0.2
[11] MASS_7.3-29 memoise_0.1 munsell_0.4.2 parallel_3.0.1 plyr_1.8
[16] proto_0.3-10 RColorBrewer_1.0-5 RCurl_1.95-4.1 reshape2_1.2.2 Rttf2pt1_1.2
[21] scales_0.2.3 stringr_0.6.2 tools_3.0.1 whisker_0.3-2