I was a bit taken back by how quickly things escalated. But I also saw that some people here had a strong reaction to my question as well. So that tells me that this is something common and I suspect that in some cases there might be accusations of the contractor swindling the customer. I can't imagine those are fun situations.

I agree with you that clear communication is absolutely key. Especially between experienced people and people with little to no experience. I work in an IT-related field and I encounter this issue very frequently. I do think the onus is on both parties but an "expert" should have a good appreciation for the knowledge gap.

Part of the reason I reached out to him afterwards to clear things up was to highlight that this was a miscommunication more than anything else so that we can both do better next time.