Shortest Code to Legitimately Slack Off
TikZ (pdfTeX 3.14159265-2.6-1.40.17), 85 79 74 24 22 21 bytes
A bunch of bytes saved thanks to wchargin
One byte saved thanks to Chris H
\input tikz
\tikz\pic

I actually encountered this one by accident when I was working on homework.  I spent quite a while waiting for it to compile before I realized what was happening.
This has two parts:
\input tikz

This loads the TikZ package
and:
\tikz\pic

This starts a \tikz environment and a draw command.
What is going on
The pdflatex compiler has trouble with \tikz\pic, and enters interactive mode causing it to stall indefinitely.