Like any other metal, thermocouples are prone to damage, oxidation and general wear and tear – this results in inaccurate temperature readings and, eventually, an open circuit and complete loss of data. Of particular note is oxidation and corrosion due to chemical attack – clean steam and peracetic acid are particularly damaging to thermocouples – and using an exposed thermocouple can lead to water and chemicals being pulled up the thermocouple sheath causing corrosion beyond the tip. For this reason when remaking thermocouples, the wire should be cut back until clean metal can be seen before the junction is created and care taken to avoid any kinks along the length of the cable.
It is also worth noting that the hot junction point is the first point of contact closest to the cold junction rather than the tip of the thermocouple. A very long twisted junction may therefore create inaccuracies of measurement, as the point of measurement will not be where you may expect it to be. Furthermore a short across the two wires (either due to damaged sheathing or overstripped wires touching in the input board) will lead to the reading being taken at the short rather than the thermocouple tip.
Finally, a loose or badly made hot junction that allows the wires at point of contact to move against each other will cause spiking, incorrect readings or repeated brief open circuit readings.