#2 Worst thing I've seen a co-worker do?

Back before we utilized stored procedures (and had an official/credentialed DBA), we used embedded/in-line SQL to fetch data from the database.

var sql = @"Select
FieldsToSelect
From
dbo.Whatever
Where
Id = @ID"

In attempts to fix database performance issues, a developer, T, started putting all the SQL on one line of code (some sql was formatted on 10+ lines to make it readable and easily copy+paste-able with SSMS)
var sql = "Select ... From...Where...etc";

His justification was putting all the SQL on one line make the code run faster.
T: "Fewer lines of code runs faster, everyone knows that."

Mgmt bought it.

This process took him a few months to complete.

When none of the effort proved to increase performance, T blamed the in-house developed ORM we were using (I wrote it, it was a simple wrapper around ADO.Net with extension methods for creating/setting parameters)

T: "Adding extra layers causes performance problems, everyone knows that."

Mgmt bought it again.

Removing the ORM, again took several months to complete.

By this time, we hired a real DBA and his focus was removing all the in-line SQL to use stored procedures, creating optimization plans, etc (stuff a real DBA does).

In the planning meetings (I was not apart of), T was selected to lead because of his coding optimization skills.
DBA: "I've been reviewing the execution plans, are all the SQL code on one line? What a mess. That has to be worst thing I ever saw."
T: "Yes, the previous developer, PaperTrail, is incompetent. If the code was written correctly the first time using stored procedures, or even formatted so people could read it, we wouldn't have all these performance problems."

DBA didn't know me (yet) and I didn't know about T's shenanigans (aka = lies) until nearly all the database perf issues were resolved and T received a recognition award for all his hard work (which also equaled a nice raise).

Hired a new BI developer. She tested reasonably ok in SQL, and certainly showed good strengths in visualising data, plus had a good attitude in the interview. We hired her. She broke her laptop the first day. We got her another then she complained the camera didn't work but didn't realise the lever in front of the camera was to move the privacy shutter off and on.

Assigned her some work of taking queries that are used in a BI tool that targets the transactional database directly, and re-jigging them for Snowflake which we're using as a data warehouse now, aggregating all our data into one place. Yet, she's struggling to understand why the SQL query she's pasted in doesn't work as-is.

I go over it again; the source schemas and tables are this, but in Snowflake we've named them this. She then bemoans how much work that is to change them all - I say use find and replace. She then struggles with Snowflake syntax errors and asks for a guide on T-SQL to Snowflake. I show her Google and say "this is what I did when I hit these problems - search for 'Snowflake equivalent to T-SQL getdate()' or 'how to get current date in Snowflake' but she still doesn't understand. I ask if she's every had to work between T-SQL and MySQL or MySQL and PostgreSQL or Oracle and so on and she says yes. I say the syntax isn't the same, is it? And she goes oh, now I understand.

She scored reasonably in her SQL test but I'm now concerned there's something fundamental missing in her grasp of SQL. I gave her a detailed demo of the tools, I explained in the interview and on her start about our move to a data warehouse for all our apps, and put her through some training plus gave her time to work through our Confluence pages - not expecting she'll remember everything, but more to ensure she recalls they exist and what the general contents are.

Anyhow, that's my rant.