You should take care to correctly set the PATH variable. Otherwise, you can accidentally run a program that was introduced by someone else. The intruding program can corrupt your data or harm your system. This kind of program, which creates a security hazard, is referred to as a Trojan horse. For example, a substitute su program could be placed in a public directory where you, as system administrator, might run the substitute program. Such a script would look just like the regular su command. Because the script removes itself after execution, you would have little evidence to show that you have actually run a Trojan horse.
The PATH variable is automatically set at login time. The path is set through the startup files: .login, .profile, and .cshrc. When you set up the user search path so that the current directory (.) comes last, you are protected from running this type of Trojan horse. The PATH variable for superuser should not include the current directory at all.