Alternate titles I've considered for this topic are:
Undeveloped TI BASIC feature?
Tokens in TI BASIC?
Sprites in TI BASIC.
Fault prediction results
Failure analysis experiments
Keyword entry feature
Anyway, going way back, it seems like I became intrigued with finding oversights in games, software, hardware and systems. Such as the trick in Donkey Kong where you can trap the fires by jumping over a peg as the fire passes under you. The Pac-Man trick where you eat a blue monster as it emerges form the center box, causing it to go off the map, than eating it again traping its "spirit" in a loop. The "hiding" places where the monsters can't find you, ever. Many phone hacks, etc.
My first TI 99/4a was a floor model, bought for me on X-mas eve, it was the last one, as I was told. Price $1000. The store made them sign a no-refund disclaimer, saying that it was a prototype. Though it did seem to run all software, in some ways it differed slightly. I clearly recall the title screen copyright 1979.
String variables Such as: A$="full four lines of text", if concatenated as such: B$=A$&A$&A$&A$&A$&A$&A$&A$&A$&A$&A$&A$&A$&A$&A$&A$&A$&A$"
typing PRINT B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$&B$ from immediate mode(not in a program), would print out proceedurally until fully complete, possibly locking the user out for hours or even days if a third string is assigned. I could not duplicate this behavior on any other TI.
O.K. now that the preamble is over!
By trying many unlikely things, I discovered that typing control or undefined function keys in a REM statement from TI BASIC causes RESERVED WORDS to print out if the program line is LISTED. Some of the function keys cause strange character sequences to print, with long delays in between(FCTN V,?).
Some reserved words, such as RANDOMIZE (CTRL U) are long enough that attempting to edit a full four line entry of them, results in a single program line, that is longer than the screen table, apparently causing an overwrite of VDP tables and perhaps causing BASIC to alter VDP registers as well. While entering chars on such a long line, sprites appear at times, also the screen can loose horizontal sync, probably from external video being activated (this will cause newer monitors to cut out).
If Function 1 (del) is the first key edit, the entire screen scrolls left! using FUNCTION 2 (ins) and entering full lenth edits can cause basic to become crazy, returning messages such as *BAD NAME IN LINE NUMER 44736(RUN), trying to RESEQUENCE such a program causes a strange pattern lockup.
Using Function 4 (CLEAR) to leave edit mode can yeild different results than ENTER.
Could this be something intended to work like the keystrokes on a timex1000
or maybe something interacting from modem control codes?