Probably the most annoying bug I ever encountered was in Quark Xpress 7 when in a two-column layout, placing a discretionary hyphen in a paragraph on the right-hand column caused a preceding paragraph on the left-hand column to reflow to a different number of lines. I tried to get Quark to look into it (it was perfectly repeatable) --- when they wouldn't, I pulled a couple of all-nighters over a weekend and shifted the entire project to InDesign.

That said, as a counter-point, there were also applications such as WriteNow which was ~100,000 lines of hand-coded assembly, which was quick and performant even on an 8MHz 68000 Mac Plus --- it flew on a 25MHz NeXT Cube (it was originally developed as a possible advanced MacWrite/replacement for it if it didn't ship, then was contracted by NeXT to make a bundled word-processor)

https://www.folklore.org/The_Grand_Unified_Model_The_Finder....