It isn't that computers are "stupid", rather they are very good at doing exactly what the programmer or end-user tells the to do. And, it is often a multitude of programmers, and administrators, and end users.

In this case, many of the symbols have special significance with file names.

Somebody chose to create avatar file names based on the user name, and also to parse the file names using a period (extension), for some reason.

Then all the testing ignored the case with user-names ending with a period.

And so, now 2 users have done the unexpected, and found a system bug. Just feel lucky it didn't cause a meltdown of the entire system.