biggest blopper on TNG

They forgot that sometimes. Like when a snowball flew out of the holodeck and hit Cap'n P.

Sometimes, you can. Replicator functions can operate within a Holodeck environment, so real items can be created as part of the program as well as the holographic stuff. How do you think participants eat in the Holodeck for a very long time?

No. The biggest blooper was firing a phaser through the torpedo bay.