The table example:
This is an interesting choice for the DM to address. The goblin's arrow would pass right through the table (and reveal it to be an illusion if it hit it), but the goblin, if it believes the illusion to be real, would try to avoid hitting it. How I'd handle it: Treat it both as if it provides cover and as if it did not, and then figure out if it is hit by the attack.
* If the attack hits even if we considered there to be cover, the arrow hits the target and bypasses the illusion, leaving it intact. The goblin aimed around the table and made his shot.
* If the attack hits the normal AC, but would have missed if we give cover from the table, the arrow passes through the table and reveals it to be an illusion.
* If the attack misses, there would be a percentage chance it hits the illusion and reveals it to be an illusion. I'd make it about 25% as the arrow could go right, left, above or below (equal chance of each - or 25% chance of each) where it was aimed. The goblin would be aiming around a side of the table, so only attacks that accidentally go more towards the table than the other directions would pass through it.
As for the pit example: People are happy to hop over a 5' pit. A 5' pit with a spring loaded spear covered in goo tends to scare them more....
The way I rule this spell:
Once cast, the illusion springs into existence and looks real to everyone except the caster.
However, anyone that understands illusions that sees it spring into existence may doubt the reality of it. They can spend an action to study it (int check) to reveal the true nature and see through it. They might also focus in on it if the illusion looks out of place, such as when a barrel appears in an alley that is usually empty and the observer knows the alley fairly well.
However, it is usually much easier to confirm your suspicion by interacting with the illusion as that is much easier to accomplish and reveals the truth automatically.
Anyone that observes something pass through it will see it as an illusion and see right through it. Tossing a coin at it would usually require an attack versus AC 10 (but not an attack action - just your 'interact with object'). If you hit, anyone that sees the coin pass through it can from there forward see through the illusion.
If you are unaware that it is an illusion and take an action that involves the illusion, without physically interacting with it, you get that int check to determine it is an illusion.
Generally speaking, this spell works best when cast before observers arrive and in a way that does not tip off that illusions are present.
