I spent two hours with my employment agreement this weekend and came out the other side with more questions than answers, which felt appropriate.
The relevant clauses in mine (a mid-size Series C, US-based, not in California):
1. Invention assignment clause. Anything I create during employment that is 'related to the company's actual or anticipated business' belongs to them. The word 'anticipated' is doing a lot of heavy lifting there. My employer makes developer tooling. I'm building a data pipeline monitoring tool. Related? Maybe? Not obviously? I'm not sure a lawyer would be sure.
2. Conflict of interest clause. Can't work for a competitor or do anything that 'interferes with my duties.' I'm not working for a competitor. The 'interferes with duties' part is basically unenforceable as long as my work is good, in practice -- but it's there.
3. Moonlighting/outside employment clause. Mine says I need written approval for 'substantial outside employment.' I've seen some that say 'any outside employment.' Huge difference. Mine also has a carveout for open-source contributions, which I did not expect.
What I actually did: I sent a short email to my manager asking if my personal project (gave a rough description, not the code) was fine given the handbook policy. He said yes, no formal approval needed given the description. I have that in writing now.
California is actually more protective for employees here -- CA Labor Code 2870 carves out work done on personal time with personal resources, regardless of what your contract says. Worth knowing if you're there.
Main practical takeaways: read the actual clause, not the folklore. The clauses vary enormously. If yours is ambiguous and the stakes are high, spend $300-500 on a 30-minute employment attorney consult. Cheaper than finding out later.