is one of the most beloved TV shows of all time, but that doesn't mean it's perfect.

With HBO Max's  now available to stream, plenty of fans will be compelled to rewatch and reminisce about its , ,  — and, for some, its biggest flaws.

In addition to some , "Friends" had issues with continuity that can be hard to ignore to this day, from petty details (like characters having multiple different birthdays) to major emotional moments getting little to no follow-through (like Phoebe's birth parents disappearing from her life without explanation).

Here are 21 of the most egregious plot holes and inconsistencies on "Friends."

Monica's apartment number changes from five to 20.

Given that Monica's building doesn't have just one or two apartments per floor, it makes little sense for her rent-controlled sixth-floor walk-up to be apartment No. five.

It appears that the production crew noticed the same thing, because her apartment becomes No. 20 sometime during the first season.

Chandler and Joey's apartment also changes from four to 19.

Rachel says that she needs glasses, but she literally never wears glasses.

On season two, episode 15, "The One Where Ross and Rachel ... You Know," Ross and Rachel go on their first date to see a movie.

Rachel complains about the movie's subtitles, apparently because she didn't want to wear her glasses.

But never once in 10 seasons is Rachel shown wearing glasses. So either she was lying to save face about the subtitles, or she walks around with bad eyesight.

Phoebe's birth mom is introduced in a climactic series of episodes, but inexplicably disappears after season five.

The season three finale features a climactic storyline: Phoebe finds out that her birth mom is still alive.

Phoebe struggles with accepting her mom, whom she had never met or even knew existed, for a few episodes on season four — but ultimately seems very excited that she finally has a real family member to rely on.

But after all that emotional buildup, Phoebe's mom disappears from the show after season four, episode 11, "The One With Phoebe's Uterus."

She's mentioned when she sends Phoebe a fur coat on season five, episode six, "The One with the Yeti" — but she's literally never mentioned again. She's not mentioned when Phoebe gets engaged, nor is it explained why she didn't show up at her daughter's wedding. (Phoebe does mention Lily, the adoptive mother who raised her, though. Interesting.)

Phoebe's birth dad is introduced in season five, but he's never mentioned again after that one episode.

Again, Phoebe meets her birth father after an emotional buildup. She had tried to work up the courage to meet him in earlier seasons, but ends up running into him at her grandmother's funeral on season five, episode 13, "The One with Joey's Bag."

After a sweet moment between them, it seems like they'll be able to build a relationship. But he doesn't show up again, and Phoebe never even mentions him.

Phoebe also tells a few conflicting stories about her dad.

On season two, episode nine, "The One With Phoebe's Dad," Phoebe says that her father abandoned her pregnant mother.

Later, however, we discover that Frank Buffay didn't disappear until after Phoebe and her twin sister were born. 

When Phoebe finally meets Frank, he says that he was a "lousy father." He says he burned the baby formula, put their diapers on backwards, and made them cry when he sang a lullaby.

To be fair, Phoebe was a literal baby at the time and wouldn't remember any of that anyway. Also, , so it's understandable to get some details confused.

The chick and the duck also mysteriously disappear.

The chick and the duck play a major role as Chandler and Joey's beloved pets in earlier seasons. They slowly disappear from the show and make their final appearance on season six, episode 20, "The One With Mac and C.H.E.E.S.E."

The birds aren't seen again, but the duck is mentioned when it eats Rachel's face cream and gets sick on season seven, episode two, "The One with Rachel's Book." (OK, we're all thinking it now: Did Rachel's face cream kill the duck?)

Given how much Chandler and Joey loved them, it's a little strange that no one mentions it when the birds die — which is only revealed on the series finale, "The Last One." (Joey apparently believes they were sent to a farm that doesn't allow visitors.)

Was Ross an accident or a miracle baby?

After Monica and Chandler get engaged, Jack Geller is inspired to tell the story of his own proposal.

"I had gotten Judy pregnant. I still don't know how that happened," he says on season seven, episode 2, "The One With Rachel's Book.

"You don't know how that happened? Your dog thought my  was a chew toy," Judy replies.

Ross is the couple's oldest child, so this would suggest his conception was an accident.

However, 11 episodes later on the very same season, both Ross and Jack contradict this story.

When Monica says their parents love Ross more, he counters, "I was their firstborn! They thought she was barren! It's not my fault."

Later on the episode, "The One Where Rosita Dies," Jack tells Ross that he was a "medical marvel."

Ross says his birthday is in December, but later says his birthday is October 18.

On season four, episode five, "The One with Joey's New Girlfriend," Ross begins to tell Gunther that his birthday is in December.

But on season nine, episode two, "The One Where Emma Cries," he tells Joey his birthday is October 18 while filling out forms at the hospital.

Plus, if you want to get super petty about it, Ross implies on yet another episode that his birthday is in March. On season one, episode four, "The One with George Stephanopoulos," Chandler and Joey give him a ticket to a hockey game on October 20, telling him it's a birthday present — but Ross says, "Funny, my birthday was seven months ago.

Rachel is similarly confused about her birthday.

Again on season four, episode five, "The One with Joey's New Girlfriend," Rachel tells Gunther that her birthday is May 5.

But on season seven, episode 22, "The One with Chandler's Dad," Rachel gets pulled over. The cop, looking at her license, says, "You're an Aquarius, huh?" — which would mean she was born in late January or early February.

In fact, all the friends age at wildly inconsistent rates.

Ross says he's 29 in seasons three, four, and five.

Given that nearly every season features a Thanksgiving episode (except season two) and a Christmas episode (except season 10), the show implies that each season tracks one year in the friends' lives.

Ross, then, seems to age more slowly than your average man. On season three, episode 13, "The One Where Monica and Richard Are Just Friends," he tells Chandler he's 29 years old. On season four, episode nine, "The One Where They're Going to Party," all three of the guys say they're 29.

Then again, on season five, episode four, "The One Where Phoebe Hates PBS," Ross says he's about to be divorced for the second time before he turns 30 — which would make him 29.

Monica, who's the same age as Rachel, says she's 26 in season one. They turn 30 in season seven.

Monica says she's 26 years old on season one, episode 22, "The One with the Ick Factor." Somehow, she only ages four years in seven seasons.

The season seven episode "The One Where They All Turn Thirty" features flashbacks for all the characters but focuses on Rachel's 30th birthday in the present. Monica and Rachel are the same age, since they graduated high school together, so we know Monica turned 30 that same year.