Enemies, enemy shots, tears and pickups will briefly freeze in place at random intervals.
The frequency of the effect depends on Isaac's luck.
Frozen enemy shots will disappear, but are still able to inflict damage if touched.
Room clear rewards have a 25% chance to spawn a duplicate, except for chests, pills, or cards.
Notes
If Isaac exits the room before the glitching animation is completed on the drop, it will not be duplicated.
Broken Modem also affects Friendly enemies and their projectiles.
Synergies
Magneto + Options?: If Broken Modem duplicates a chest that was spawned by Options?, using Magneto to immediately open the chest while in its glitched form will create a TMTRAINER item if the chest contained an item.
Interactions
Contract From Below: Broken Modem has an independent chance to duplicate each item spawned by Contract From Below.
Options?: Chests that were an alternative reward can be duplicated. The two chests contain different items.
Broken Modem is a reference to an internet modem/box. Since the internet box is broken, there is a large amount of lag, as the client (player) computer is having trouble getting the data from the host server over the stuttering internet. This shows itself in the item through the following effects:
The client computer has trouble getting the inputs of other players over the broken internet, meaning the other clients freeze in place and look like they aren't moving, as the computer isn't getting the inputs fast enough to show them. (slows enemies, items, and tears)
The client computer gets the host's or other clients' inputs twice, as the client computer can't send the acceptance trigger to the host server fast enough. (double items)
Frozen enemy shots will disappear, but are still able to inflict damage if touched.
Magneto + 
Options?: If Broken Modem duplicates a chest that was spawned by Options?, using Magneto to immediately open the chest while in its glitched form will create a 
TMTRAINER item if the chest contained an item.
Contract From Below: Broken Modem has an independent chance to duplicate each item spawned by Contract From Below.
Broken Items 
Glitch Items 
The Binding of Isaac: Rebirth
