Bit errors in wireless communication predominantly stem from external interference as well as from multipath fading and attenuation. In order to tackle these harmful influences, the IEEE 802.15.4 standard includes time slotted channel hopping. We have collected packets containing bit errors from 200,000 packets generated in two different testbeds. We show that the channels used in IEEE 802.15.4 exhibit different error patterns typical for either external interference or multi-path fading and attenuation. These insights allow to detect, classify and quantify the presence of these phenomena. Furthermore, practical use cases for exploiting the knowledge on error patterns on a per-channel basis are presented. We propose to choose Forward Error Correction on a per channel basis and provide reference values in terms of code error correcting capability required to recover from 50 % of the occurred packet errors on certain channels.