You know the PCs rely on magic to much when....

[reveal]

This situation came up last session.

You know the PCs rely on magic to much when they travel to an overrun fort, clear it out, and then plan to head back to town. They take 10 minutes on a plan which includes the Wizard casting teleport and the two PCs he can't take with him being stuffed in a bag of holding since they can breath for at least 10 minutes and teleporting doesn't take that long. Then they look at the DM.

PCs: "That's the plan. We put these two in the bag and cast teleport. Once there, we'll take them out and..."

DM: "What about your horses?"

PCs: *blink* "So I guess we're travelling overland then."

 

[Mystery Man]

And the girls say:

"Save the horse, ride a wizard!"

 

[I'm A Banana]

Naaaaaaaah, that's not too much magic, that's a well-known adventuring tactic. In fact, the shops selling bags of holding often put that on their signs:

BUY A BAG OF HOLDING TODAY! (because you never know when you're going to need to be a piece of equipment!)

 

[Romnipotent]

"The Horses can be tied up here! We killed everything anyway, and then the wizard can go back and forth to get them"

c'mon

Relying on too much magic is when you have a magic bottle of air so you never breath that impure Material Plane junk.

 

[Lord Pendragon]

"The Horses can be tied up here! We killed everything anyway, and then the wizard can go back and forth to get them"

Even with a wide-mouth Bag of Holding you're going to have a heck of a time getting a horse inside.

For that you need a Portable Hole.