In C "x = 0" is a legal expression, so you could have if(x = 0) { ... } and it would assign 0 to x and then treat it as false. The compiler won't complain (unl

In C "x = 0" is a legal expression, so you could have if(x = 0) { ... } and it would assign 0 to x and then treat it as false. The compiler won't complain (unless you have warnings turned on, which you should). But if you do "if (0 = x)", you're assigning to a constant, which will result in a compile error. So "if (0 == x)" guards against a typo where you omit the second =. The GMail outage of 2010, where Google had to restore 10% of GMail accounts from tape backup, was precisely because of this. A C++ migration script had used the "x = 0" construct, which set the variable to false and made the script go haywire. So not a theoretical concern. The other major takeaway from this was to always treat your migration scripts like production code and use the same warnings, tests, and defensive programming that you would use everywhere.