Video:

Written guide:

https://slashinfty.wordpress.com/2016/05/14/a-guide-to-the-garbage-of-sml2-in-19-gifs/

Additionally, a better look at what's going on at the pixel pipe. The top 2 images show the exact position Mario should start running from before sliding onto the pipe. If you start running from further left there's a high risk that you will overshoot the correct position on the pipe (keep in mind that Mario has to come to a halt on a specific pixel (shown on the bottom image). You cannot influence the outcome once Mario starts sliding. So the setup is the important part here), starting further right won't give Mario enough speed to reach the right x-position. Note that you can still overshoot the right position occasionally, even from the right start points due to the random character of how accelleration is handled in SML2.