Introduction:
Parallel mode of communication refers to the simultaneous transmission of multiple bits of data over separate communication lines. It allows for faster data transfer by transmitting multiple bits at the same time. However, it also has its disadvantages, one of which is its costliness.

Explanation:
1. Cost:
Parallel mode of communication requires separate communication lines for each bit being transmitted. This means that for transmitting n bits of data, n communication lines are needed. As the number of bits being transmitted increases, the cost of implementing and maintaining these multiple communication lines also increases. This can be particularly expensive for large-scale data transmission systems.

2. Erroneous Data Transfer:
Due to the simultaneous transmission of multiple bits, parallel mode of communication is more susceptible to errors compared to serial mode of communication. Any interference or noise in the communication lines can lead to errors in the transferred data. These errors may not be easily detected or corrected, leading to inaccurate data transfer. In contrast, serial mode of communication, which transmits data bit by bit, allows for better error detection and correction mechanisms.

3. Security of Data:
Parallel mode of communication may also pose security risks. Since multiple bits are transmitted simultaneously, it becomes easier for unauthorized individuals to intercept and access the transmitted data. This can compromise the security and confidentiality of the information being transferred. Serial mode of communication, on the other hand, allows for more secure data transmission as it transmits data bit by bit, making it more difficult for unauthorized access.

Conclusion:
In conclusion, the disadvantage of using parallel mode of communication is its costliness. Implementing and maintaining multiple communication lines for simultaneous data transmission can be expensive. Additionally, parallel mode is more prone to errors and poses security risks compared to serial mode of communication. Therefore, while parallel mode offers faster data transfer, it is important to consider these disadvantages before deciding to use it in a communication system.