One of the mistakes that many programmers repeat when they learn to program is not initialize correctly the objects's attributes. What are the attributes? They are character's features that can be modified in the execution of a project, like their position, their size, their color, their orientation...

When you have blocks than modify character's feature, you should always have a program which, when the program begins, asigns the value that we want to start. For example, if it modifies the position of a character, we should have the block 'go to' under the block 'when green flag clicked', in order to place the character in its initial position.

                when [right arrow v] key pressed //Not initialized correctly.
                change x by (10)
                when [left arrow v] key pressed
                change x by (-10)
            

                when [right arrow v] key pressed  //Initialized correctly.
                change x by (10)
                when [left arrow v] key pressed
                change x by (-10)
                when green flag clicked
                go to x:(0) y:(0)