What happened in the following screenshot?

A permissions error would produce a clear 'Permission denied' message rather than garbled or binary output.

A buffer overflow would cause the reading program to crash or behave unexpectedly, not display binary characters from a log file.

The /var/log/btmp file stores failed authentication attempts in the binary utmp structure, not as plain ASCII text. Attempting to display it with a command like cat produces unreadable characters; the correct tool to parse it is lastb, which interprets the binary format and renders it as human-readable output.

While log tampering is a real attacker technique, the garbled display here is the expected, normal result of misreading a binary-format file with a text viewer.