In the 1980s we downloaded games from the radio

I definitely had cassette based games on the TRS-80, but most of the "wireless" transmission in my youth was via BASIC printed in the back of computer magazines. You had to type in the entire app yourself. I did this for basically every app they listed. Sometimes it was like tax prep software, but I didn't care, even though I was like 9 at the time. Yes, it took a very long time. Yes, you could easily introduce typos and bugs.

Sometimes the typos were in the magazine itself, and you wouldn't figure out the problem with the code you triple-checked you typed in properly until the errata in next month's issue :)

The compiler/interpreter couldn’t even tell you what line the error was on!

You’d just get a big error message for the whole program.

After a while, magazines like Commodore Run and Compute started including a short program that would checksum each line as you entered it, so you could check that against a checksum in the magazine. Of course, you had to get that program typed in correctly first before you could use it to enter others.

I'm curious, can you say more about "as you entered it"?

Do you mean like, "for lines 1-20 the checksum should be 0xDEADBEEF"? This would let you find the error before finishing the program.

Or just at the end, it would checksum the whole thing?