Most likely nothing to be concerned about. Toasters employ a thermocouple to stop the operation once the set degree of toasting is reached - this is in fact tied to heating up. The thermocouple's shape changes and the cage in which the bread sits is released - it pops up because it is spring loaded. At the same time the electrical connection is broken or else the elements would continue to glow. These things are about the simplest gadget in your house other than a door bell system (the wired type). When the electrical circuit is broken a spark can be (often is) created at the moment of separation. Toasters are usually of sufficient wattage (consumption rating) that the "off" action (and sometimes even the "on") generates a clearly visible spark once they have been used for a while (associated with very small wear on the contacts). This will often be an intermittent occurrence so more obvious at times than at others but in my opinion of no concern at all. Some are, however, terrified of this new concept of what is called electricity and so wary of it that they are in a constant state of anxiety. In fact, regulations exist which have even led some electricians to apply earthing measures to plastic piping. That said, have a look to see if the power cord is entirely intact and everything concerning the plug is as it should be.