Understand how computers handle different data types and what their min/max values are. Someone sent me the following code today (in Javascript):

ord=Math.random()*10000000000000000;

A) It's unnecessary and will get "trimmed" down to 2147483647 on 32-bit systems. B) Clearly the programmer that wrote this has absolutely no idea that his number is going to require a 64-bit long int (on applicable systems) just to generate a random number to make their URL non-cached. What a waste.

</minor rant>