client-side-validation

I saw here that: As you probably already know, relying on client-side validation alone is a very bad idea. Always perform appropriate server-side validation as well. Could you explain why server-side validation is a must? Client-side validation - I assume you are talking about web pages here - relies on JavaScript . JavaScript powered validation can be turned off in the user's browser, fail due to a scripting error, or be maliciously circumvented without much effort. Also, the whole process of form submission can be faked. Therefore, there is never a guarantee that what arrives server side, is