This happens to us all at one point or another.

You're typing in something long and complex, like a URL, or a person's name, and without even thinking, you hit tab and expect the details to be filled in. I have done this with

• URLs
  
• Passwords
  
• People's names
  
• My own street address
  
• Various words of greater than five letters in length.
  
• Node titles
  

Another, similar phenomenon is handle substitution. I'm known as getzburg in many circles, and I've caught myself signing things with "getzburg" twice. Thank God they weren't tax forms....