I realized that I'm uploading code every 5 minutes and doing relatively intensive trial-and-error development, and that this habit could cause problems down the road, especially if I'm working on a custom board (non-Arduino) where the chip isn't readily replaceable.
What will happen?
I noticed that avrdude verifies the written flash. So will it simply start noticing errors and possibly be unable to write without failing?
Or will it work sometimes, sometimes not?
Example of heavy use: A custom PCB where easy replacement of the MCU is not possible. 10,000 writes would equal 100 days, each 8 hours of development, with flash write every 5 minutes.
I have several habits that drive frequent flash writing: I inject benchmarks into my code, test speed improvements of small optimizations, program size optimizations, and so on.
I now try to restrain myself from flashing too frequently and doing more code revisions before testing, rather than testing immediately.
In conclusion: Yes, it's unlikely, and arguably, if you're working full-time on a board, less intensively, for, say, a year, you can probably afford to buy another board when the first one starts failing the flash-verification.