Psilocybe cubensis mushrooms vary in potency quite a bit depending on strain and growing medium.
If grown on straw they look beautiful and are very large, but extremely weak.
If grown on rye grain, they can be extremely potent.
SWIM has had some of his own home grown psilocybe cubensis that were almost as potent as psilocybe azurescens.
The main two reasons for having a bad trip with shrooms are:
1 - you took too much. It's very easy to take too much. You may have taken a batch from one source that were weak and then assume your next batch from a different grower is the same potency so you double your dose only to find that your new batch is 10 times more potent. Be careful. SWIM overdosed on psilocybe cubensis once, eating a large batch of mushrooms containing approximately 200 mg of psilocybin. They were cooked in water, so SWIM thought they would not work (he believed the rumors that boiling them made them inactive), but they worked just as good as raw ones. It was not something SWIM would ever want to repeat. Within 5 minutes after eating the mushroom soup he was tripping harder than he had ever tripped before or since. It was scary as all hell. SWIM loves mushrooms, but at that dose, I can't see anyone liking it. The following day SWIM had a few broken blood vessels here and there in his arms. Very high doses of psilocybin interfere with blood clotting.
2 - they are old. Something about their effects changes if they are not refrigerated and kept in an airtight container. SWIM had some he grew himself that he forgot to put in an airtight container. They were sitting around for a few years. SWIM tried them and the effects were awful, very stimulating, made him get a little twitchy. I don't know what happens to them when they are not properly stored, but it's definitely not the same anymore.