Descriptive Problem Summary:
This bug is also known as the "Invincible Bug".

This is where a player either Logs in or Relogs right before a new match refreshes the player list and logs in as a spectator yet is chosen as a player, and when the match starts, the other player can literally walk through them and cannot attack them while the bugged player can attack the other player.

This bug is only a one-match duration, and after the match is over, any other matches they will do will not have the bug in effect, and if they relog again, they cannot access the bug until above conditions are met.

Numbered Steps to Reproduce Problem:

1) Wait for a match to end.
2) Relog
3) Choose a character
4) INVINCIBILITY

Expected Results:

Both players have a density of one and can attack each other.

Actual Results:

One player is invincible with zero density.

Does the problem occur:
Every time? Or how often? Every time.
In other games? Not unless they stole your code, Bravo.
In other user accounts? Of course.
On other computers? YES.

When does the problem NOT occur?

When the person doesn't log in at that inconvenient timing.

Did the problem NOT occur in any earlier versions? If so, what was the last version that worked? Before you added Jiraiya

Workarounds:

Have them relog.