We fixed 15+ bugs in our stock analysis engine: here's what was wrong and how we fixed it

We've been running a stock analysis app for a few months and recently did a deep audit of the analysis engine. Found some embarrassing bugs and fixed them all. Sharing for transparency and in case anyone building similar tools finds it useful.

**The worst ones:**

**1. Fibonacci levels were mathematically wrong**
The support/resistance levels were calculated with two different formulas that happened to produce the same number. Result: the same price level appeared in both supports AND resistances simultaneously. Classic copy-paste error that went unnoticed because the output "looked reasonable."

**2. Unprofitable companies scored as "Excellent"**
A company with negative ROE and negative margins was getting a health score of 81/100. Fixed with explicit handling: negative ROE → -15pts, negative margin → -10pts.

**3. Long-term Monte Carlo scenarios were inverted**
The "Base" scenario was showing better returns than the "Optimistic" scenario. This was caused by historical drift being used instead of a fundamental-based drift, causing percentile ranges to cluster incorrectly.

**4. 2-year CAGR was using a 3-year lookback**
We were using `iloc[0]` (oldest data point, \~3 years back) with an exponent of `1/2` (assuming 2 years). Fixed to `iloc[-504]` (504 trading days ≈ 2 years from the end).

**Other fixes:**

* PE ratio negative values showing as "cheap" valuation
* "Approaching Death Cross" treated the same as confirmed Death Cross in recommendations
* Analyst price targets returning 6 decimal places ($892.119284 instead of $892.12)
* ROE >50% now shows real value with context note: "possibly amplified by share buybacks"
* Strengths/weaknesses limit raised from 5 to 7

**Lesson learned:** Always test with edge cases, unprofitable companies, stocks at 52-week highs/lows, and non-US tickers. Most bugs only showed up when we manually compared outputs across multiple real tickers.

If you want can check on: -stocksanalyzer.app-