Invalid frames are a normal by-product of packet capture. When we capture packets, we can understand most of them, but some of them are garbled and don't pass the CRC checksum that we do to make sure that the packet is good.
It's similar to eavesdropping on a conversation. You can understand most of it, but sometimes you have to fill in the blanks on your own. Fortunately, the human brain is pretty good at doing that. Wi-Fi chipsets? No so much. 
I'd be okay with a 25 percent invalid rate, although physically moving closer to the devices and checking for interference with a spectrum analyzer would both be good steps to improve it.
Joel, Mobility+, ECSE, CWNE #233
Technical Trainer
MetaGeek
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