Having been denied a 10th-minute opener following a video review, the first period got worse for Tappara in the final minute, when Jere Karjalainen gave IFK Helsinki the lead.
But the side sitting atop the table seemed to have found their form in the middle period, as Joachim Blichfeld equalised before Benjamin Rautiainen turned the match around going into the final 20 minutes.
However, the side beginning the night in 14th went on the offensive in the last period. Sean Josling levelled the scores in the eighth minute, then a high-sticking call against Jonathan Andersson was used to full effect by HIFK, as Daniel Makiaho's powerplay goal put them ahead again.
Rather than defend their lead, the capital club kept attacking, as Otto Somppi added another with nine minutes to play, then Anttoni Uronen made it 5-2 with less than six minutes to play.
Juuso Nykanen scored to give Tappara hope with three minutes left, but that was extinguished 90 seconds later, as Petrus Palmu eased the puck in the empty net to give the Helsinki side a 6-3 win, their third in a row and fifth in the last six, moving up into 12th place.
Tappara remain top of the table, but the gap remains two points following Assat's win last night.
It's now eight wins in nine for KooKoo, who have climbed above JYP into third with a 5-2 win at home to KalPa, putting a four-point cushion between themselves and those outside the top four.
KalPa had the last word in the opening two periods to only trail 3-2 with 20 minutes to play. Ossi-Petteri Jaakola re-opened a two-goal lead early in the third but this time KalPa were unable to respond, and Ari Grondahl ended the contest with a late empty-netter.
The champions from Kuopio have now slipped to eighth following successive domestic losses.
Lukko are now the best of the rest, moving up a place to fifth by coming from 2-1 down to win 4-2 at home to Karpat. It's three wins on the spin for the Rauma side, while Karpat stay one from bottom following successive losses.
Kiekko-Espoo were in need of a win after going three matches without one, and a home match against Hameenlinna - one of the only sides in worse form - proved the ideal tonic, as the hosts held on for a 3-1 win and condemn the visitors to a fifth defeat in a row.
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