Some registration scenario.
1. Use a program to generate a unique and strong password.
2. Copy the password to the clipboard.
3. Paste the password into the text input field for passwords.
4. (The pasted text is silently truncated.)
4. Continue the registration process.
5. Attempt to log in.
If the password was longer than the maxlength value set for the text input field, then it was silently truncated.