1 - Dueling in the wrong lobby
If you were to spectate a game, then attempt to join/host a game lobby, you will be taken to the game lobby you last spectated. Sadly, the game lobby would be glitched, and there is a probability of you getting kicked out of the server, requiring your to refresh the page.
2 - Frozen Deck
When playing against a deck that is very search heavy; there is a chance of you not being able to move your cards. You'll be able to right-click them, but you can't drag them around, and right-clicking them then selecting a command won't do anything. The chat log would be updated as if you successfully conducted these moves for some reason. To fix this, quit the game lobby.
3 - Invisible Cards
Some decks are glitched for whatever reason, causing them to not show in-game for other players. Only the owner (and sometimes spectators) will be able to see the affected cards. Moving a card that is "invisible" will cause the owner's chat log to be updated with the action conducted using the card, but it won't show for the opponent. To fix this, reveal any number of cards, then quickly shuffle your hand.
4 - Ghosts
If you enter Duel Portal's "Duel" lobby, you'll probably notice a huge amount of players hosting. You can request to duel them, and wait till they relog. It seems like anyone who afks in the duel lobby and never relogs causes this "ghost" glitch. Basically, the in-game name is there, but the player is offline. You're never going to get an answer since the players aren't there. These "ghosts" are eventually removed when DP goes down for maintenance. Until then, simply ignore them.
5 - Cheat Portal
Now, this is one of the more severe bugs. In the latest update, a zoom-in feature was added. When hovering your mouse over a recently added card, you could see their full card art. This seemed like neat mechanic at first. But then, people started realizing that it wasn't programmed well into the game. Lets say your opponent moved one of their face-down cards to another slot on the field (Maybe they're OCD as hell or whatever). Well, by hovering your mouse over the slot that previously contained the once face-down card; you'll be able to see a zoom-in of the card, as if it was face-up on the field.
These are some of the glitches I found on DP ^ Just wanted to list them down.