Help! Berry plant full of mushrooms!

[treedimensional]

The spores have gotten into that pot somehow. How does the plant look? If it's okay, blame the soil, and repot the plant using fresh (soil-free) potting mix. If the plant looks bad, throw away everything and start over from scratch.

[treedimensional]

I suppose it's possible, but doesn't seem likely. I would guess that spores landed on your pot after you planted it.
Potting mix is different than soil. Most potting mix contains no soil at all and is produced under sterile conditions, so I don't think it was a bad bag to begin with.
If the plant looks okay, you may not need to do anything; but it depends on what kind of mushrooms you have. The Arborist in me is just hypervigilant about root rot, and that's probably just as unlikely. I shouldn't have been so drastic.