What happens if I exceed 183 days mid-assignment in Israel?

Once you cross 183 days in a single tax year, you are presumed to be an Israeli tax resident for that entire year under Section 1 of the Israeli Income Tax Ordinance — meaning Israel can tax your worldwide income retroactively for the whole year. The presumption can theoretically be rebutted by showing your center of life remained abroad, but this is extremely difficult in practice. Any treaty exemption your employer was relying on also evaporates. If an assignment is likely to approach 183 days, engage an Israeli tax adviser well before you hit the threshold.