This assert (and same for width & height) semi-randomly hits on debug built unity (maybe a console message on release) and it's annoying. This backtrace and instance is on Mac 10.15.5 and OpenGL, but I've hit this on metal and I think D3D11 too (back when we were in office).

Version: 2020.2.0a14.1750
Revision: trunk 15dd1dcb0579

Note that one frame above:
// TODO: on win/dx backbuffer surface extents are not updated, so assert will be hit
if (depthSurf && !depthSurf->backBuffer)
VALIDATE_CLIENT_RS(depthSurf);

That's in GfxDeviceWorker.cpp line 1108. The graphics command is: kGfxCmd_SetRenderTarget

(lldb) bt
* thread #76, name = 'UnityGfxDeviceWorker', stop reason = EXC_BREAKPOINT (code=EXC_I386_BPT, subcode=0x0)
* frame #0: 0x0000000107972587 Unity`ValidateClientRenderSurface(rs=0x0000000122c17640) at GfxDeviceWorker.cpp:57:5
frame #1: 0x00000001079249c2 Unity`GfxDeviceWorker::RunCommand(this=0x0000000122c26c90, stream=0x000000018d1c0080) at GfxDeviceWorker.cpp:1109:21
frame #2: 0x00000001079707db Unity`GfxDeviceWorker::RunExt(this=0x0000000122c26c90, stream=0x000000018d1c0080) at GfxDeviceWorker.cpp:417:5
frame #3: 0x000000010797008a Unity`GfxDeviceWorker::Run(this=0x0000000122c26c90) at GfxDeviceWorker.cpp:401:5
frame #4: 0x00000001078f94d5 Unity`GfxDeviceWorker::RunGfxDeviceWorker(data=0x0000000122c26c90) at GfxDeviceWorker.cpp:380:42
frame #5: 0x00000001030deb7d Unity`Thread::RunThreadWrapper(ptr=0x000000012619e1e0) at Thread.cpp:78:14
frame #6: 0x0000000121538c65 libsystem_pthread.dylib`_pthread_start + 148
frame #7: 0x00000001215344af libsystem_pthread.dylib`thread_start + 15