Why do my keyboard Ctrl and Shift keys randomly fail?

Here are the symptoms: When pressing Shift or Ctrl as a modifier, they sometimes don't work. Like I'll try and capitalize a letter and it will be lowercase, or I'll press Ctrl + V and instead of the pasted text, it'll just paste 'v'. Additionally, sometimes it will stop working while I'm holding it down. Like I'm going to right now hold down shift and press a character, and this is what I get: 'IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIiiiiiiiiiiiii'. It just stopped working. Didn't move my finger or change pressure or anything. I assumed this was the fault of my keyboard, and so I bought a new one. And

Question by dgo on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.