Probably, you haven't ever paid attention on this moment.
Even professional players make this mistake pretty often. The best example – RpK any ZywOo. Those guy replenish armor for nothing at least several times each event they play in.
In fact, you mustn't re-buy your armor+helmet if you have
• 50 or more points left playing as T
• 57 or more points left playing as CT
Otherwise you just waste $650.
It may not cause the instant consequences, but in the long term a couple of such mistakes might break your economy.
For example, you unnecesserly re-but the armor in the third round ($650), then you do it also in the fifth round ($650 more).
So, let's say, you wasted $1300 when you had 5-rounds winning streak and then, after losing 2 rounds in a row, and suddenly you have $3000. Oops.
It's an easy mechanical tip that will help you to have a better economy.
SCOPE.GG allows you to track the mistake in the Dashboard section below retake stats.
In the end, you can replenish armor when you have a bit lower numbers than 57/50 – you won't lose the maximum possible amount of armor points every round.