In a more general sense:
- as tempting as it is, never promise anything
- saying no, is a very important thing
- as soon as a sentence starts with "that's easy you just have to" run for cover
- it's okay to ask for help, it doesn't make you look incompetent and if it does you're in the wrong team.
- if you feel uncomfortable .... just leave
As a backend programmer:
- how does time and space complexity work
- different datatypes and strategies
- know the difference between comfortable with and the right tool for the job
- stateless architectures and atomic design are better in the long rund
- code for the domain not for yourself -> if it's a tool for admins write it with configs, if it's for devs write it with a database, .... etc.
As a frontend programmer:
- only a few people know CSS most of us just guess
- declarative is the stable solution if you want to have less work
- don't couple layout to the DOM structure, couple them to classes
- fancy JS is awesome but it comes with a price.
- don't do hype-driven-development :)
I can go on but I already cheated with categories ;D ....