This comes up every time we try and lint the r-devel sources, e.g.
get_source_expressions('src/library/grid/vignettes/grid.Rnw')$error
# src/library/grid/vignettes/grid.Rnw:81:1: error: [NA] Missing chunk end for chunk (maybe starting at line 81).
# <<eval=FALSE>>=
# ^
This comes up every time we try and lint the r-devel sources, e.g.