I recently landed a CL which worked fine on all platforms except Mac, due to some different network behavior there.  The bots didn't catch it because browser_tests didn't even run on the OSX bots (See https://chromium-review.googlesource.com/c/chromium/src/+/1197924 / https://ci.chromium.org/p/chromium/builders/luci.chromium.try/mac_chromium_rel_ng/131949), though they did run on other platforms.

I'm not sure if the browser_tests on OSX are just disabled on the bots now or if modifying code in chrome/test/ppapi/ppapi_browsertest.cc and ppapi/tests/ (Which in fact contains browser_test-only code) doesn't trigger browser_tests on OSX.  Whichever is the case, it still seems to be happening on the OSX tryjob for the re-land (https://chromium-review.googlesource.com/c/chromium/src/+/1202242).

Not sure about the label for this sort of issue, or where the relevant python code lives.