Clockwork Swarm

Oracle Text

Clockwork Swarm enters the battlefield with four +1/+0 counters on it.

Clockwork Swarm can't be blocked by Walls.

At end of combat, if Clockwork Swarm attacked or blocked this combat, remove a +1/+0 counter from it.

X, T: Put up to X +1/+0 counters on Clockwork Swarm. This ability can't cause the total number of +1/+0 counters on Clockwork Swarm to be greater than four. Activate this ability only during your upkeep.

Card Rulings

10/4/2004 Loses a counter even if it is affected by a Fog-like effect which prevents it from dealing damage.
10/4/2004 Can attack or block even if it has no counters.
10/4/2004 If the ability to add counters resolves when there are already the maximum number of counters on it, any counters over the maximum are simply not added.