Is it fine to drop the ball at work? That depends on what kind of balls. Everyone makes mistakes, we are human. It's best that you have an honest, constructive discussion with your manager before it leads to critical business issues. But what are the signs that you should speak out? How do you even broach the subject with your manager?

Hell Yes!
If you are starting out in your career, it’s always a good idea to say “hell yes!” to every
opportunity that gets thrown your way. If you are in your first 12 months of career, focus on what’s given to you and ask for more. You are starting out, so all projects are platforms to hone your skills and build your network. Develop a system for you to stay on top of your tasks. Build your personal brand as a credible contributor.
Obvious Signs To Speak Out
What if you are 18 months into the role, you are entrusted with quite a few responsibilities but your plate is so full, that anything more is just going to cause indigestion?
Then I think the best thing you could do is to bring this up to your team leader and prioritize together.
There are some obvious signs that you are at a stage to have that discussion with your team leader.
You started dropping some balls
You are working till late each day
You feel overly exhausted everyday
You might want to continue owning everything given to you just because “it’s yours”. Some projects are like your babies, you’ve been caring for it since its infancy. It’s hard to let it go.
So you think it’s fine to be part of 10 different things. It’s normal to be busy anyway. Your manager would understand that sometimes you might let some things drop. My question to you is...
What’s your goal? Is it to deliver; or somewhat deliver?
Definition of Being Productive
Don’t mistake being busy with being productive. Production means you are creating, growing, manufacturing and improving. Being productive means you have actual output. It is your responsibility to be productive, not be busy.
By dropping the balls, you are being the bottleneck in production lines, directly or indirectly. As employees, it’s our responsibility to deliver.
With that in mind, it is perfectly fine for you to have a productive discussion with your team leader. My suggestion is for you to start thinking deeply about these points first:
You list out all the things on your plate now
You list out all the things you are dropping now
Reflect hard on these question:
What are the gaps? Processes? Technical skills? Challenging stakeholders?
Are there better ways to do things more efficiently and within my control?
What can my team leader do to help me?
Then have an honest discussion with your team leader. Because you have reflected on all the questions, you are going to your team leader with solutions on top of the challenges you might have faced.
Need a script?
One way to broach the subject is “I have quite a few things on my plate but I noticed that I am not doing well in <<insert>>. I’d like to have a discussion with you on how to manage better. I have some solutions that I’d like to share, it’ll be great if you could give me your thoughts…”
Ultimately the organization works as a team to deliver. Do see it as your responsibility to seek a solution. If you are stuck, it means that you'd need some help. Do make it your ownership to seek the support that you need to be successful.