Several languages offer the eval() function and its safety is hotly debated. You hear eval() is only one letter away from evil()!
My feelings, if eval() is directly connected to a user input, avoid it!!! This is particularly true for Python's input() function that uses raw_input() and then sends the string to eval() to extract the number. In this case you can't even intercept the input, so if a nasty person enters (don't even try this!!!) "__import__('os').system('del *.*')" you just wiped out all the files on your drive!
If you use raw_input(), then you could check the input string for 'os' or 'system' before sending it to the eval() function. A safe use of eval() would be in a GUI calculator, where the input is limited by the buttons you have supplied.