waiwai933 ran into this on English.SE, and I've confirmed - attempting to suspend a user for longer than 999 days results in a message being sent, but the user left unsuspended.
Steps to reproduce
- Select "contact user privately (includes suspension)" from a user's mod menu
- Select a template
- Check the "also suspend this user" checkbox
- Enter 2555 in the "for ____ days" entryfield
- Press the "Notify and Suspend" button.
- Observe that a message has been sent and the user's account annotated, but not suspended.
The obvious behavior would be to suspend the user for the specified number of days. However, if there is a limitation on suspension length, this should be noted in the UI and enforced in a less confusing manner: display an error, or at very least, prevent the form from being submitted until a valid number is entered.
Tim Stone notes in comments that this bug has been posted before, and sure enough, this does appear to be describing the same behavior. However, I would like to emphasize the confusing, potentially-harmful nature of sending a message to the user being suspended while failing to actually suspend them: if unnoticed, this could be rather counter-productive.
If deletion is the desired action for users who are violating the ToS and have no expectation of being allowed back on the site in any reasonable amount of time, then this should also be noted in the UI.