Miranda, a professional decorator, recommends a "17th Century grand piano" for Stuart's B&B. The piano is an 18th Century invention, while the grand piano, as we know it, was not invented until the late 19th Century. What did exist in the 17th Century (which in fact resembled a piano) was a harpsichord.
When Daniel goes to his brother's place and they go through the makeup process to turn him into Mrs. Doubtfire, they go through several iterations until they finally reach one that is acceptable as the character. Other than to showcase Robin Williams' comedic talents, there was no reason to make him up to look like a Hispanic woman and then Barbara Streisand. They should have focused on creating a look that would fit the persona that Daniel created while on the phone with Miranda.
When Mrs. Doubtfire throws the lime at Stu, it cuts to Stu getting hit in the head. When it cuts back to Mrs. Doubtfire, the same lime is clearly at the top of the fruit dish. The same lime disappears and reappears from the fruit dish several times in this scene.
When Mrs. Doubtfire and Miranda are unloading a paper bag of groceries, Mrs. Doubtfire pulls out a small bag of tomatoes. In the next shot, she pulls out the same bag of vegetables and carries them to the refrigerator to place into the crisper.
When Mrs. Doubtfire serves tea to the Inspector who visits, her home, the amount of tea in the Inspector's cup changes after the camera cuts away to show the melting cake fall off Mrs. Doubtfire's face into the cup.
The first time Daniel emerges from the ladies room and asks the maitre d' for a menu, the knot of his tie is skewed to his left. In the next shot, when he reaches Mr. Lundy's table, it has been straightened.
When Mrs. Doubtfire goes into the kitchen, she closes a regular door in the middle of the hall. Throughout the rest of this scene, there are two swinging doors to the kitchen, and they are located at the other end of the hall.
In the birthday party scene at the beginning, Daniel sees Miranda and jumps off the table. When he takes his cap off, his hair is flattened down. After Miranda pulls the plug from the outlet, the next shot is of Daniel with his hair neatly combed.
In the apartment inspection scene, when Mrs. Doubtfire pulls her head out of the refrigerator, there is icing on the wig. After showing the inspector and cutting back to Mrs. Doubtfire, the wig is clean.
When Daniel hastily changes back into Mrs. Doubtfire for the social worker, he jerks on his Mrs. Doubtfire wig and leaves it crooked, with a portion of his brown sideburns sticking out. But in the next cut, when he's creeping up the hall to the kitchen, the wig is not only straight but appears to have been pinned neatly into place.
During the sequence where Daniel is calling Miranda with horrible "potential nannies", he makes one call while in a bathtub. Between shots where it cuts between him and Miranda, the bubbles on his body change position.
The "Mrs. Doubtfire" face that Daniel wears is not a mask, but a prosthetic appliance, as evident by its uneven edges. These have to be carefully glued down and blended with each use. It can't simply be put on and removed like a mask.
During the sequence where Daniel is calling Miranda with horrible "potential nannies", Miranda is seen responding on a cell phone, home phone, and her work phone. It is entirely possible that Miranda has "Call Forwarding" from her home phone to her cell while she is out. The two numbers published may have been her home and work numbers.
When Daniel is putting in his false teeth, you can see that some lipstick got on his teeth. This is a normal problem, even on real women with real teeth. It should also be noted that Daniel was not sober at the time.
In the pool scene, Stu chats with Ron who knows him well enough to know about Stu's dating habits. After Mrs. Doubtfire throws the lime at Stu and claims an unhappy kitchen staff member was the culprit, he hears Mrs. Doubtfire chuckle and call Stu a loser. Ron looks at her and Mrs. Doubtfire speaks in a man's voice again, threateningly to him. A friend or acquaintance would have brought that up immediately or confronted the old woman with a younger man's voice.
The reason Daniel did not lie to Miranda to get out of the Bridges dinner was because his youngest daughter begged him (as Mrs. Doubtfire) to go, and he couldn't face disappointing her. There is an entire scene devoted to demonstrating that, and it's the reason he agreed to go in the first place.
When Mrs. Doubtfire first meets with Miranda and is making tea for them, she fills the tea kettle with water and puts it on the stove, but the kettle never boils and is not seen pouring it for tea anyway, there is also no steam coming out of the cups.
The goof items below may give away important plot points.
When Mrs Doubtfire gets her own children's TV show at the end, she asks one of the puppet characters if he knows anything about England. The reply is 'I only know that it's an island', which isn't true. Britain is an island, not England.
When Stu is at the restaurant we see him place a shrimp in his mouth, and chew it thoroughly. Moments later he begins to choke and Mrs. Doubtfire helps him. But when he coughs out the shrimp, it is whole.
After Chris and Lydia learn that Mrs. Doubtfire is a male, Lydia threatens "her" by picking up a tennis racket as a weapon. However, the next time Lydia is shown in a full-body shot, she is no longer holding the tennis racket.
When Mrs Doubtfire gets her own children's TV show at the end, she asks Kovacs, "Do you know what language they speak in England", and Kovacs says "Pakistani?". Pakistani is not a language, the national language of Pakistan is Urdu (among other provincial languages like Punjabi, Sindhi, Pushto, etc..) The word Pakistani only refers to the people and culture of Pakistan, there is no language by the name of Pakistani. But the whole point of the scene is that Kovacs gets everything wrong.