In Acts 2:3, the Holy Spirit came what seemed to be tongues of fire:
3 They saw what seemed to be tongues of fire that separated and came to rest on each of them.
This observable phenomenon only mentioned once in Acts. Subsequently, all records were plain.
Acts 8:17 - Then Peter and John placed their hands on them, and they received the Holy Spirit.
Acts 10:44 - While Peter was still speaking these words, the Holy Spirit came on all who heard the message.
Acts 19:6 - When Paul placed his hands on them, the Holy Spirit came on them, and they spoke in tongues and prophesied. (NIV)
Why would the Holy Spirit's coming at Pentecost be different to other occasions when people received the Holy Spirit?