(For background, you may want to read about the time I’ve spent trying to get my T-Mobile account working as expected.)
Today I tried to pay my phone bill and noticed that the T-Mobile website account associated with my current number had been deleted (presumably this happened when my plan was cancelled). This was unexpected, but only mildly frustrating. I went through the registration process, generated a new password in my password safe, and pasted it into the field.
Nothing happened. They were preventing pasting, presumably as some kind of demented ‘security’ measure.