The high-flying Springbok-laden Sharks outfit hosted 15th-ranked Zebre at Kings Park this weekend, where they were heavy favourites to come out on top.
However, the plucky Italian club took advantage of a plethora of Sharks errors, scoring four tries and led the contest going into the 75th minute.
A last-gasp Ethan Hooker try and a successful conversion from Jordan Hendrikse would see the hosts earn a narrow 35-34 win over Zebre.
The Sharks struggled to hold on to the ball - coughing up 23 turnovers - and kicked the ball away aimlessly.
"It doesn't matter who you are playing against, if your skillsets are poor, you can't build pressure with the ball, you don't play in the right areas, your kicking is not good and you play against a side with a full back like they have [Geronimo Prisciantelli] you will be in for a long day," Plumtree told the media.
"Our turnovers allowed their backs to hurt us. We made it easy for them with our [poor] skills, which made it hard for us. We were up for it emotionally, but our skills were terrible and it almost cost us the game.
"They will be hard on themselves. The team will be hard on themselves. We have a lot of Springboks out there that are very proud. We will put our hands up and said this wasn't good enough.
"As coaches and management, we have to look at what we could have done better also."
Next up for the Sharks are Irish giants Leinster, who are coming off the back of their first loss of the URC season against the Bulls, who picked their pocket with a winning penalty in the 84th minute.
Plumtree is under no illusions as to the challenge that awaits this weekend.
"We have a good side to play against next week, we know we can't afford to play like that," he added.
"They will punish us and won't let us off. They won't give us the soft moments Zebre gave us.
"We will have to play a lot better in every aspect of the game. We have a lot of work to do this week and be better next week."