When replacing guitar strings, never do so anywhere near an amplifier (especially a valve amp), nor close to a mains outlet. Because the strings are thin - the top "E" string in particular - they can easily work their way into mains outlets, ventilation slots and all manner of tiny crevices. The springiness of the strings means that they are not easily controlled until firmly attached at both ends. This is very real - the image below is an actual photo of an Australian mains plug that was shorted out by a guitar string. Luckily for the guitarist, both active and neutral were shorted together, blowing the house fuse. Had the string only contacted the active, the guitar and guitarist would both be live. Touching any earthed metal could easily have resulted in a fatal electric shock.

This is a real photo, taken of a mains plug that had a guitar string across the active and neutral pins. Needless to say, the plug had to be replaced, and it must have made an almighty flash when it happened.

Modern Australian plugs have an insulated section that covers the first 8mm of the AC pins, so just the final 8mm of the pin is metal, allowing for contact within the socket.  This prevents this kind of danger from happening, but many countries have no such protection.  The requirement for the insulated section is legislated, and any new plug that doesn't include it is prohibited.  Mains leads and plugs are so-called 'prescribed' or 'declared', and must have regulatory approval and bear the 'regulatory compliance mark' (RCM).  It's illegal to sell any mains lead that is not approved.