I have created a new column for my Announcement App, and I defined the “Number of lines for editing: ” = 1. But when adding new item to the announcement, user can enter more than one line for this new column ? Any idea why is this happening ?
That doesn't have any effect on how much content a user can enter, it only effects how large the input field is rendered on the new/edit pages. If you want to limit the amount of text, you'd either have to set it as a single line of text field (which caps at 255 characters) or create a validation sccript that checks for a max number of characters and prevents the item from being saved if it exceeds it.