Even the best of DMs with the best of players can have the occasional off-night. Whether it’s simply the players taking an unplanned direction while the DM draws a blank, or external stresses bubbling over and causing an argument, no campaign goes off without ever having a problem.
There is, however, a difference between a minor problem and a game-ending problem. A minor problem may conceivably lead to your game being cancelled for the evening to give the DM time to think up a way for the adventure to continue in the unexpected direction (or even to let tempers cool) but a major problem could cause players to permanently leave the game, or even end friendships. Most major problems begin as minor problems that, through improper handling or neglect, develop into a serious issue.