Hmm… Interesting, I'm seeing the same thing. For similicity's sake, I think the exact replication of the example using linejoin = "mitre" from the example is probably most useful to show. I'm also going to use reprex to show the problem (always a good way to go):
# Control line join parameters
library(ggplot2)
df <- data.frame(x = 1:3, y = c(4, 1, 9))
base <- ggplot(df, aes(x, y))
base + geom_path(size = 10, linejoin = "mitre", lineend = "butt")
Created on 2018-01-13 by the reprex package (v0.1.1.9000).