Grocery delivery start-up Instacart has discovered a bug in its checkout flow that affected some of its users' ability to waive service fees and some shoppers' ability to receive tips.

In a blog post, the company says the bug was related to product updates made at the end of 2017 and fixed this week.

It's a rare slip-up at one of the most prominent unicorn companies in tech, at a time when it's getting ready to battle Amazon in the grocery delivery space.

Instacart is increasingly being seen as big chain grocers' answer to the Amazon/Whole Foods threat. The company has partnered with six of the top seven grocers in North America and just raised a fresh round of funding that values the company at $4.2 billion.

This mistake may raise questions about whether Instacart has the technology or ability to compete with a behemoth like Amazon at this level.

In the blog post, Instacart founder and CEO Apoorva Mehta apologizes and says the company takes "any issue like this very seriously and we are rectifying it."

He says payments will be issued to all shoppers who were impacted by the bug and refunds will be issued to customers who were charged service fees incorrectly.