Being one of the most popular and extensively studied ERP-BCI paradigm, ERP-spellers are commonly built and tested in ideal lab settings. However, in practical applications, users may encounter complex situations and various mental processing that have been believed to affect ERP signals. This kind of effect will probably induce a deteriorated performance of ERP-spellers. In the current study, a working memory task was interleaved within an RC ERP-speller paradigm to examine the effect of concurrent mental processing both on ERPs evoked by the stimuli of speller and its performance, especially when a speller is built under pure lab setting but used under complex mental activities in real life. The results show that the amplitude of N200, P360 and N550 were significantly affected by the working memory task. Moreover, the performance of ERP-spellers was significantly deteriorated by concurrently performing a working memory task, not only when a speller is trained and used in different settings, but also when a speller is both built and used in the same complex setting. These findings introduce a challenge for ERP-spellers to be used outside lab-settings and in daily work, especially when users are undergoing complex mental processing and experiencing heavy mental workload.