  A bit catastrophe, loosely defined, is when a change in just one character of
a string causes a significant change in the size of the compressed string. We
study this phenomenon for the Burrows-Wheeler Transform (BWT), a string
transform at the heart of several of the most popular compressors and aligners
today. The parameter determining the size of the compressed data is the number
of equal-letter runs of the BWT, commonly denoted $r$.
  We exhibit infinite families of strings in which insertion, deletion, resp.
substitution of one character increases $r$ from constant to $\Theta(\log n)$,
where $n$ is the length of the string. These strings can be interpreted both as
examples for an increase by a multiplicative or an additive $\Theta(\log
n)$-factor. As regards multiplicative factor, they attain the upper bound given
by Akagi, Funakoshi, and Inenaga [Inf & Comput. 2023] of $O(\log n \log r)$,
since here $r=O(1)$.
  We then give examples of strings in which insertion, deletion, resp.
substitution of a character increases $r$ by a $\Theta(\sqrt{n})$ additive
factor. These strings significantly improve the best known lower bound for an
additive factor of $\Omega(\log n)$ [Giuliani et al., SOFSEM 2021].
