Comment out lines matching pattern with Sed

Let's say you've got a file with the following lines:

cat file.txt 
leave me alone
leave me alone
comment me out
leave me alone
comment me out also
leave me alone

Comment out the lines that are begging for it:

sed -i '/comment me out/s/^/#/g' file.txt

Voila:

cat file.txt 
leave me alone
leave me alone
#comment me out
leave me alone
#comment me out also
leave me alone

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