Nearly everyone that needs to know this already knows but maybe some still don't!
During a period in the last 25-30 years, the exact time depends on the specific manufacturer, a yellowish glue type of compound was used to secure components to printed circuit boards. It's recognisable by the fact that its a bit stretchy if its pulled and can be peeled off with care using a pair of fine nosed pliers. It's also recognisable because where it comes into contact with hot components it changes colour. Not too hot it goes darkish yellow, a bit hotter it goes darker and very hot it goes nearly black. At some point between darkish yellow and black the glue decomposes and by-products eat away tinning and copper converting metal to a green verdigris-like chemical. During the transition phase the resistance of the glue changes from "good insulator" to "good conductor". Interesting things start to happen as extra resistances are added willy nilly to areas of circuit. Initially intermittent events will occur then hard faults appear and finally wires, component leads and even circuit track will disappear and faults are compounded one upon the other. The dark glue can be chipped away. It's lost its elasticity and gone very brittle. Once removed clean up the affected area with a strong solvent such as MEK and repair any open circuits.
The last occasion I saw this was the last week of June 2000 in a very expensive car amplifier made by "Longmill". It's used in TVs, VCRs, Radios and virtually anything electronic.