Acid trap was the one I kept hearing, and proved true on my home-etched PCBs. The 90-degree traces would get undercut but the 45-degree ones were fine. To be f

Acid trap was the one I kept hearing, and proved true on my home-etched PCBs. The 90-degree traces would get undercut but the 45-degree ones were fine. To be fair it's less likely to be an issue with more advanced etching processes, but there are still limits. The thicker the copper layer, the wider your minimum track and gap widths become, otherwise the copper traces are excessively undercut.