((NOTE: This was fixed in patch 3.02. Critting off the bonus Q damage is no longer possible.))
For those that aren't familiar, Nasus Q has always been able to crit, however the crit only modified your BASE damage, and then your Q damage was added on top of it.
However, I just confirmed that Nasus Q is now critting off ALL of your damage, including your base AD, base Q damage, AND bonus Q damage. It is not clear just now exactly when this change went through, as it was not in any patch notes, and we have no word from Riot on the matter yet.
Previously when you would crit on a Nasus Q with a Tforce:
(150 * 2) + 225 + 110 + 400 = 1035
(That is, 150 base AD * 2 = the crit + 225 from your tforce proc + 110 from Q base + 400 bonus Q damage)
NOW WHEN YOU CRIT ON NASUS Q WITH A TFORCE:
(150 * 2) + 225 + (110 * 2) + (400 * 2) = 1545
See this screen shot posted by lavalampe on the EU bug report forums a week ago:
This shows 99 base damage, 110 base Q damage, and 144 Q bonus damage being added up to 353. 100% crit modifier means * 2 making it 353 * 2 = 706
Previously this crit would give you 452 damage because the crit would modify your base AD and then your Q damage would be added after the fact.
You can also see it live in this video posted by TheNomiraven if you suspect the image is shopped:
Do the exact same as above... add up the base AD, base Q, and bonus Q. Multiply by all of it by 2.
This was not in the patch notes, and I have no idea why/when the change slipped through, or if it is an intentional ghost change or a bug that is going to be reverted back. However, if it is not reverted this could have pretty massive implications for Nasus builds. You could literally be seeing Nasus critting for over 2k on Qs. It is hard to say the practical implication this actually has for Nasus, because trying to build crit going to be tough with the way he needs to itemize (tanky/CDR oriented), but you can bet your ass people are going to have some fun with this and look for ways to abuse.
Edit: Please don't post in here arguing that you could always get 1k + crits on Nasus. I know that. I know the Q attack has always been able to crit. But it was your base damage critting + tforce effect + Q bonus added on after the fact. Yes it was a big number. But now EVERYTHING is criting and it is going to be MUCH BIGGER.