With HP Procurve switches it is easy even for a novice to clear the user password. When physically at the switch you can press and hold “Clear” button with a paperclip. You know, that one you keep in the drawer for poping open CD/DVD ROMs? By doing that you have reset password to the configured default which is admin with no password. NOTE: Beware, if you press “Reset” and “Clear” at the same time this will reset the switch to factory settings. This applies to at least the HP Procurve 25xx series, 26xx and 28XX.
Anyone who manages HP Procurve devices knows that at some point you will not be able to access it because it does not recognize the username / password. Better yet the last admin didn’t have it recorded for his replacement. 
If you are unsure please contact HP or consult your user manual (downloadable from HP).
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