lecture crashing 

VERB
The act of attending a university lecture for a class you do not take.
Derivative of gate crash

Reasons behind this behaviour vary... One could lecture crash to keep a friend company, to hide the fact that they have no one to hang out with during a break, to try before you buy, or maybe just because they find the subject interesting.

"Hey Clementine! What are you doing here?! I didn't know you took psych 101!"

"Hey Fatima, nahh I don't, I'm just lecture crashing"

lecture crashing by RoboBear September 20, 2011

lecture crushing 

VERB
The act of attending lectures for a class you don't take because you are crushing on someone there--either another pupil, or the lecturer (in the case of the latter the 'crush' could be completely platonic--perhaps admiration of teaching/oratory skills).

Derivative of lecture crashing

"Bro, that hilarious philosophy lecturer we had first year is taking a class this semester! We are sooooooooo lecture crushing!"

lecture crushing by RoboBear September 20, 2011