EV3 Brick Error Symbols?
I know that this question is old, but if someone else has this question, I have found that these error messages often appear when you are either trying to access a non existent array (i.e. you copied the block from another project and you did not change the name of the variable) OR when trying to access a value of an array with an index that does not exist (i.e. In an array of size 4 you are trying to access index #4 or index #5)