When asking people to fill in the fields, be clear on the format of the answer you expect. Specify what characters can be used, and what characters can’t be used.
Don’t wait to correct people until after they’ve clicked continue, because you’ll be using up your visitor’s supply of patience. Advise them at the outset of the right process. Try combining this with AJAX error notifications that don’t wait until people click continue, in order to be more efficient.
The more you dip into that reservoir of patience, the fewer conversions you get.
If you require people to use both letters and numbers in their passwords, say so in the form. If you want it a certain length, say so.
Password must include numbers and letters – why didn’t you say so to begin with??
Similarly, how was I supposed to know that a field labelled “City / State / Country” would not accept forward-slashes?
You’re begging for slashes by naming the field like that – it suggests the format people should answer in.
Ironically, in the very same form, I saw the following very clear, and commendable instructions:
Fill in your name as it appears on your passport. Clear. Concise. Avoids nicknames, initials and other abbreviations. Effective!