What have you learned the hard way about sales tax compliance as your business grew across state lines?

As our business started selling in more states, sales tax got a lot more complicated than we expected. I’m curious to hear from others who’ve been through this. What parts of sales tax compliance surprised you or caused problems as your business grew across state lines?

Were there mistakes you made early on or things you wish you had understood sooner? I’m hoping to learn from people who’ve already gone through it and help others avoid the same headaches.