Defective Weapons Inc.

[Bombadil]

Ents storm Orthanc, break dam, not strong enough to withstand water, all get washed away.

That doesn't have to do with a weapon, but a mishap nonetheless.

Grima: I hate you!
*Throws palantir, hits somehting springy and bounces back up hits him in the face*
*the present members of the fellowship point and laugh.*

Bard: My black arrow has never failed me!
*Breaks in half as he pulls it from quiver*

or

Bard: My black arrow never fails to hit a target!
*thrush whispers in ear*
*Bard shoots arrow as thrush flies away, accidentally hits thrush.*
Bard: uhhh...oops?
*Turns around and walks away whistling*

*Isildur travelling with party of men*
*Orcs ambush them, Isildur puts on ring to disappear*
*As he swims away invisible, ring slips off finger*
*Orcs see Isildur, shoot him to his death*

Oh wait...that actually did happen