“Spellings” in Katakana aren’t always consistent.

ワイングラス： a wine glass

In this word, “glass” is rendered as グラス, pronounced gu-ra-su. However:

窓ガラス： window glass

Here, “glass” is written ガラス, which is pronounced ga-ra-su.

This has no real conversational consequences, especially if you talk quickly. However, the practice section in my textbook quizzed me on the proper form of both words, which means I probably need to remember the differences in how “glass” is rendered for the test. Oh, how deliciously evil!