When AJ sings "What you want..." Nick is wiping his nose on his sleeve.
In a few scenes there is a sandwich board in the background, it reads "Backstreet Boys, Everybody Welcome."
When AJ is singing "yeah..yeah" Howie is totally out of it, he is in his own little world.
Towards the end when the guys are dancing on the stage, some girls have a lighter and when Kevin kicks the lighter goes out.

Get Down

when Nick sings "I'm in delusion.." Brian is making weird faces behind his back
in the second chorus when they sing "You're my ecstacy " Kevin is singing the wrong words.
in the second chorus when they are singing "Get Down..Get down.." Howie is doing the wrong dance and then he looks back once he realizes he's doing the wrong moves.
when the sing "whoa...whoa" Brian misses the clap

I'll Never Break your Heart

when the girl that Brian is after, goes to pick up the rose she is wearing Brians jacket.
In the same scene, when they sing "to show how much....." the girl has on a different jacket.

Quit Playin' Games(with my heart)

when Brian sings "Deep within my soul.." his watch slips down his arm.
when Aj is singing "Baby, baby..." Brian is coughing in the Background.
when AJ sings " The love that we had was so strong, don't....) Brian is doing a weird little dance in the background.

Everybody(Backstreet's Back) the short version

In the beginning skit, Brian and Kevin are the only one's with bags, I guess the rest of the guys don't plan on changing their clothes in the morning.
when AJ sings " Now throw your hands up..." the girl on the right in the back is way off.
At the end dance scene when they sing "Rock your body right..." Brian is just sitting on the floor looking dumb, while the other guys are dancing.

Everybody(Backstreet's Back) the long version

in the beginning dance scene Nick starts singing too early
In the first chorus when they sing Everybody for the first time, Kevin sings "Rock your body.." before the rest of the guys do.
In both of the dance scenes watch Nick one second he's in front of the girl dancer, the next minute he's behind her. Nicky keeps jumping around.

All I Have to Give

There are two different versions to this video, one was released only in America. There are some differences, like for one, they took the water dancing scenes out, along with at the end when the special effects make it look like they are dancing out of each other. Also in Howies solo, there are two scenes when he is in the sweater, instead of one as before.
Aj lounges in chairs a lot in this video
At the first chorus when Brian is singing " But my love is all...." it looks like the guys are zipping up their pants.
At the last seconds of the video when Brian sings "thats all I have to give"

I Want it That Way

Kevin

In the 2nd chorus during the dance, Kevin looks really weird like he missed a step or something.
When Nicky sings "Don't Wanna hear you say" after Howie sings, Kevin does a spin and looks like he's getting ready to fall
At the last"....mistake, Tell me why" Kevin gives the camera a really cute face.

Howie

Throughout the 1st and 2nd chorus Howie looks completely out of it, every once in a while he will do a cute little dance.
At the fan scene Howie does a dance while walking backwards and he almost runs into AJ
When AJ is singing the last line Howie taps Nicky on the back to tell him something

Brian

After he sings the opening line, when the bass hits, Brian does a cute little dance with his head.
When AJ sings "You are my fire... the one desire" in the second verse Brian looks at his watch.

AJ

AJ must have his seasons mixed up, cuz all the other guys have on jackets and he has on a tank top
in the 2nd chorus when they sing"....nothing but a mistake" Aj looks like he's dying from lack of oxygen.
After 2nd chorus when Bri is singing " I Never wanna hear you say...." AJ is singing the wrong words, I dunno what he's saying but it's wrong.

Nick

In the 1st few seconds of the video, after AJ sings "Yeah" as Nick walks through the airport, he's laughing at something.
As Brian sings opening line ".....desire, believe.." Nick has this devilish grin on his face.

The Whole Group

In the second chorus Brian's voice is the only one we hear that says " I never wanna hear you say.." but the entire groups lips are moving.

I'll Never Break Your Heart (98 US Version)

When Aj sings "As time goes..." Brian's hair goes flying up in the back
When Aj sings "Get to know me..." Howie and Nick mouth along with him.
On the second "No way, no how.." Kevin grabs his face (Oh how dramatic) - Before the final chorus when there is a repetition of beats Howie jumps around like a rabbit.