This didn't derail the game, it's also in-game rather than meta, but it still makes me shake my head. Most of my table is completely new to, or very inexperienced with RPGs. The one experienced player, unfortunately, had to recently leave the group, otherwise this never would have happened.
So the characters bid adieu to the cleric that the experienced player ran and need to get from City A to City B, 800 miles away, across the sparely inhabited grasslands. I figure we'll do a session of travel (we have 3-hour sessions). The players spend over an hour in real time preparing for the journey. I tried nudging them along a couple of times, but they really enjoyed the planning. Oddly, the thought that horses would be a good idea, was the last thing they thought of. So after 2/3 of session they set off on their 3 riding horses and a pony (for the gnome warlock). They even add a warhorse after a brief encounter with some brigands.
At the start of the next session, when they get near the coast, just for some color, they encounter patches of quicksand. Their passive perception fails and the gnome and pony walk right into some quicksand and after rolling badly, (something this player does constantly due to his steadfast refusal to stop using dice that must be weighted rather unfavorably) he and his pony sink like a stone. The monk throws a rope to the gnome. The monk, while mounted, tries to pull the gnome free from the quicksand, using just her arms. This fails and the gnome disappears under the quicksand. The monk then ties the rope to her saddle and uses the horse to drag the gnome free. Fortunately, because of the extra horse, the gnome still has a mount.
The players decide that the bard will now lead, and tie a rope around him for safety. (You see where this is going, right?) The bard fails a perception roll and walks into more quicksand. He tries to turn the warhorse around and rolls for the horse but botches… badly (a 0.5% chance at our table). The horse breaks its leg, but the bard is easily rescued do to the aforementioned safety rope. The bard player asks if he can use his sword to probe the ground, and I say “certainly”. At this point the gnome player realizes he has a staff that would be quite useful here.
So they lose a pony and a warhorse, but gain valuable experience regarding marshy terrain. The gnome now shares the bard’s mount.
Fast forward to the end of the session. They’ve picked up an emaciated escaped slave who begs them to take him back to City B, where he escaped from. (He’s afraid of the monsters and is starving in the wilderness and life was much better as a kitchen slave.) The party randomly encounters some griffons. I glance at the MM, and notice the text says they like to eat horses. None of us had realized just how much the players had leaned on the cleric during combat. The encounter should not have been a difficult, especially as the party was rested. Due to poor tactics, the griffons start wearing the characters down and the characters are doing almost nothing in return. The bard and warlock fall off their horse, so I use the opportunity to thin the ranks of the opponent. One of the griffons grabs the horse and flies away, followed by another. This doesn’t improve things. By the end of the encounter the players are all making death saves (none die), their horses are gone, and there’s an escaped slave in the middle of it all wondering, WTF?
Epilogue: The slave died in the next session after another particularly bad encounter. The monk player said that at least he was able to die with a full belly. To which the warlock player replied, “A belly full of arrows.”