Even people who aren't familiar with The Lord of the Rings have heard the name Gandalf and know something about his character. A powerful and influential wizard, he was known throughout Middle-earth and had various names. Although he was of divine origin, that doesn't mean that he was perfect, and Gandalf made a few mistakes in the course of his long and interesting life.
Gandalf had already existed for thousands of years before the story of The Lord of the Rings takes place and had many amazing adventures before the Shire as Bilbo knows it even existed. Considering what he accomplished, it's easier to forgive some of his more shocking and disappointing errors.
7 Refusing To Leave Valinor
Gandalf had a lot of nerve being so hard on Bilbo for not wanting to leave Bag End, considering that his own master had to push his reluctant servant out the door with some force. During the ancient history of Arda, Gandalf's name was Olórin, and he took orders from the highest Vala, Manwë, and the radiant spirit was happy where he was and didn't want to leave his homeland.
Olórin was ordered to leave Valinor and travel to Middle-earth to fight against Sauron and Morgoth, but initially refused, openly admitting that he felt too weak and was afraid. Manwë insisted that was the best reason to make him go, and Olórin's reluctance is why he was the last of the Valar to reach Middle-earth.
6 Graffiti
This is the worst kind of graffiti, too. Gandalf does more than write a glowing rune for all in the night to see, he etches it into the door of the Baggins house, making it impossible to paint over or remove.
There's no detail about how the rune was removed, but it must have been at some point, otherwise, wayward dwarves would be stopping at Bag End as if it was a hostel. Bilbo's fortune would have bought new doors for the whole house, but that doesn't justify Gandalf defacing the door of a venerated heritage home of such cultural and social importance.
5 Uninvited And Voracious Guests
As if defacing Bilbo's home wasn't enough, the whole idea of marking the door with a rune was to signal Thorin's company to the location as not only a place to stay but a place where they could make themselves at home and behave accordingly. That means an empty pantry and a stressed-out hobbit as the host.
Dwarves and Hobbits have similar interests when it comes to food and hospitality, and get along fairly well, but the initial shock of the dwarvish onslaught on his private home was almost too much for Bilbo Baggins. Gandalf was having so much fun messing with Bilbo that he didn't put enough thought into this, but luckily the Took side of Bilbo that salvages what might have been an even more difficult evening.
4 Too Trusting Of Saruman
Everyone has heard the stories about Gandlaf's wisdom and insight, but if he's so smart how did he miss one of the most obvious traitors in all of literary history? It's not as if they just met, either. Saurman and Gandalf go way back, to the days of Valinor, when the former was known as Curumo. During the time of the War of the Ring, and he was considered the leader of the Istari, also known as Wizards.
Curumo was a servant of Aulë, the Vala who was known for amazing skill when it came to craftsmanship and invention. However, he seemed to have a bad track record when it came to his pupils and servants going rogue. Sauron was also one of his students before he betrayed the Valar and sided with Melkor, also known as Morgoth. So how did Gandalf miss that Saruman was going to turn traitor?
3 Too Trusting Of Butterbur
Even less trustworthy than Saruman but for a whole set of completely different reasons, it would seem that a Wizard would have a better, more secure way of sending messages, like using a familiar, a courier, or something to that effect. Gandalf, on the other hand, is just going to leave this letter, which contains information that may be a matter of life and death, with the innkeeper.
Granted, Barliman Butterbur isn't a total rando, and at least he's a stable person with a business to run. That might have been Gandalf's naive motivation, but that's exactly what makes Butterbur the wrong person to trust with an important message. He's a busy guy with too much to do already.
2 Leaving The Dwarves And Bilbo In Mirkwood
When Gandalf leaves the Dwarves to their own devices near the beginning of the journey, it's understandable that he might think they could function for a few hours in a familiar bit of wilderness without him. When they get mixed up with the trolls due more to their greed and stupidity than anything else, and Gandalf saves them, it's easy to side with the Wizard.
The next time Gandalf leaves Bilbo and the Dwarves alone, however, he couldn't have picked a worse possible time. Mirkwood had a pleasant history, but he must have known that by the time Bilbo and the dwarves arrived, it was a dangerous place. He leaves them with a map and tells them to follow the path, but says next to nothing about the spiders, the Wood Elves, or how the whole wood messes with the average mind.
1 Sniffing Through Moria
The strangest thing about Moria is that it's one of the most famous locations in Middle-earth and was one of the continent's major cities for more than 1000 years, but nobody seems to have any idea of the floor plan. It might have been wise for Gandalf to prepare better in case the path of the Fellowship led through the dangerous mines, but there was too much fun to be had in Rivendell.
Gandalf guessed his way through most of the mines, knowing little more than the other members of the party, and the bridge of Kazad-Dum was a famous landmark that wasn't exactly a secret. It was mostly luck that got the Fellowship through the mines as opposed to Gandalf following his nose, but there's also his dramatic sacrifice to consider, so all is forgiven in this case.