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PSA for a FAQ
If you can't power up your mon, it's because they're on a team. Take them off, do the necessary and re-add. (This is actually on Niantic's public buglist.)
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I wish they could have coded it so if you put a mon on a team in a league, you can't power it up beyond that league's limit. I powered up quite a lot yesterday, and it would have helped my peace of mind if I could have put one in the ultra league, for example, and then only been able to power it up to the max for that league. For every one, I was afraid of either getting distracted or fat-fingering that final power up confirmation and going to 2502 CP.