- The system can't handle more than 250 volumes.
- We have to ship by August 31, no matter what.
- We've chosen to use the singleton pattern here.
- No double-byte characters allowed in login names.
Now, this seems really obnoxious. We're telling users "no" and "can't", either directly or indirectly. Although we complain about these constraints (and our users do, too!), constraints are good for users, in general. Without constraints, users have no guidance. Telling users what they can't do is tantamount to telling them what they can do.
So remember, constraints are okay. What's not okay is not knowing your constraints. So find the boundaries of your system and tell users what they are. Feel free to live by the rules!