This is because && and || are often used as conditionals: [ -e README ] && cat README avoids an error if the file README doesn't exist, and [ -e READ

This is because && and || are often used as conditionals: [ -e README ] && cat README avoids an error if the file README doesn't exist, and [ -e README ] || echo "You should write a README!" works the opposite way. What's more pernicious is that pipelines don't cause the shell to exit (assuming set -e) unless the last command fails: grep foo README | sort does not fail if README doesn't exist, unless you've also used `set -o pipefail`.