I just clicked the "Hire for 5 gold" button under the Magic Catcher button a handful of times without paying attention, and nothing seemed to happen so I didn't mind it. It apparently gave me a new magic bomb for every click, though, and I tested it again purposefully to see if clicking hire had done it, and found out it was the cause.

In short: Clicking Hire for magic catcher while not having gold produced a Magic Bomb in my inventory, and didn't subtract currency.

Edit: Also, I now have 10 magic bombs when I should only have one.

Thanks for reporting!

The bug is fixed. You can leave the additional Magic Bombs you got from this glitch as a reward.