this is a very serious matter that I learned in a very hard way, So let me share my experience regarding this.
During my finals in school I had a project supervisor that disturbed us seriously on the amount of money we should pay for my project work. We paid paid the amount he told us and when we finished the the whole project work, a day to our submission he came up with this random 15k that we should pay before ever signing our work. We begged him but he refused and that led us not to do our internal defence. That made me very angry at him and at several occasion  I will make unguided statement behind his back about how wicked the man his and on how he treated us very bad. The department latter allowed us to joined in the final defense and while we where waiting for out turn for the defence, conversation came up and I repeated the same statement that he is a bad man man and I made statement like " that man chop our money wetin no good" after I made that statement I looked behind me and saw that he was standing just next to me. I was shocked and speechless and even had to run away because I didn't know how to react at that moment and that was when my sense cane back to me that I was being too unguided with how I use my thong.  Till date I still have some things I needed him to help me with but because of what I said, I I don't even know how to approach him anymore.

I know once the thought gets to the mind, it becomes very difficult not to utter it out but we should be very careful on what we say in the public because the walls truly have ears.