I've actually had this experience once. My first time investing, i didn't factor in the county/school tax in the area and was only looking at the local city tax, which was an additional $1000 per year. Ultimately the cash flow was still really good, but I did try to fight the county on the taxes - had a representative do a re-appraisal. At the end of the day, it still didn't matter because they gave every excuse to justify the tax $$ was correct.
After that, I typically am extremely conservative when I look at opportunities and do as much research as possible. When modeling these opportunities, I would use 5% of purchase price - even though it may not be realistic in different areas.
Keep in mind please, that if you are buying the property at a premium to a prior sale - i.e. paying $100K vs previously sold at $60K - the taxes may actually increase, because the government may actually use your price as a justified appraisal value.