I wrote Windows for the TI-81.

This is a great idea!

I used If inString(Str1,Str2) and inString(Str2,Str1) instead of If Str1=Str2.

1) I used the product() and sum() commands to access List elements instead of L1(N)

Just a few days ago, I had been using the combination sum(mid(list,n,1)) instead of list[n] to access list elements on an Nspire.

I once made a 20 KB game in which I had over a hundred labels, many of which used If-Then statements with no End. Actually, I didn't know how to use End back then.

And come to think of it, I actually did that seven times -- six times with 20 KB text-based RPGs, and once with a Jeopardy imitation (called Leopardy) that wouldn't let you get halfway through the game because of the memory leak

Okay, the following is a VERY VERY dumb thing to do. Don't try it.

When I first got my calculator and discovered TI-BASIC, I immediately started working on that 20 KB program described above. I didn't know about groups at the time, so I saved it from RAM clears by archiving. Needless to say, I quickly found my first GarbageCollection message staring me in my face. Unlike some other newb programmers who would instinctively know it's a bad thing (which is the right idea), I thought it was interesting, and after finding that it was caused by unarchiving and archiving repeatedly, I did that, just to see the
Garbage
Collecting…
message several times a day, every day (idiot).

I think it took me about 6 months, but I'm not sure anymore. If I recall, I discovered For( first, so I had stuff in my code such as

For(Z,0,1
0->Z
code
If <goes outside the screen>
Goto LBL
End

Or stuff like that

Then I discovered about While/Repeat

One of my first programs was to get revenge on some kid. I wanted to make his calculator never turn off so that his batteries would last a few days. I went through the flash debugger and searched for every single 76h (halt). Then I made a program to replace them all with zeros. Here is the source:

     LD     A,00
     LD     ($02A3),A
     LD     ($08E9),A
     LD     ($0AA8),A
     LD     ($3E81),A
     LD     ($3F8F),A
     LD     ($3F53),A
     LD     ($3E68),A
     LD     ($3683),A
     LD     ($362F),A
     LD     ($3629),A
     LD     ($3623),A
     LD     ($361D),A
     LD     ($3533),A
     LD     ($3527),A
     LD     ($347F),A
     LD     ($32F9),A
     LD     ($32F3),A
     LD     ($312B),A
     LD     ($2F51),A
     LD     ($2F14),A
     LD     ($2EBA),A
     LD     ($2E79),A
     LD     ($2E5B),A
     LD     ($2E55),A
     LD     ($2E4F),A
     LD     ($2736),A
     RET
Needless to say, it didn't really work right. Not only did he not have an 83+, or os 1.16. But I don't think you can change the OS that easily.

When I first got my calc, I programmed Counting programs constantly (start at 0, and add one each time.  They later got more advanced, and after it was above 10, it would start adding 2, after 100, 3, after 1000, 4 (etc))  I would bring my calc into Mellow Mushroom (a pizza/italian place) and work on my counting programs there ^^

Then I wrote a program that converted between metric units, standard lengths, standard volume, standard weight, temperature, and time.  It ended up being over 9000!! err actually, about 9000 bytes, and was filled with labels, gotos, if Condition:Do something blocks, My metric conversion part looked like:
if StartUnit = "K"
1000->A
if StartUnit = "H"
100->A
if StartUnit = "D"
10->A
...
if EndUnit = "K"
1000->B
if EndUnit = "H"
100->B
if EndUnit = "D"
10->B
...
StartNumber * A / B ->EndNumber

All my friends were crazy about text based rpgs using over 9000 labels/gotos, I offered to make a nice looking menu system so they just have to do "Text To Display ->Str1:PrgmMENU and it would do all the menus for them, but they insisted on Disp "Line 1":Disp "Line 2":Disp "Line3":PrgmGETKEY:If A=1:goto AA:If A=2:Goto BB...etc...