Is the padding wrong on the "Length Extension Attack" Wikipedia page example?
50- byte padding staring with 1 and rest is zero ( each \ is a byte)
\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00

8-byte size part of padding where the size is encoded ( SHA-1 has 64-bit length encoding in the padding that makes 8-byte)
\x00\x00\x00\x00\x00\x00\x02\x28

0x228 = 552 and we have 69 bytes of message size.
That is correct. 55-byte message and 14-byte secret make a 69-byte message.
The message in bits makes 552 bits that are over one block of SHA-1 that has a 512-bit block size. So in total, we need 1024 bits for hashing, two blocks. 64 bits reserved for the message size. This left 960 bits. Now subtract the 552-bit message size and we have 408 bits. So we need 51 bytes of padding starting with 0x80 but we have 50, i.e. $\color{red}{\text{1-byte is missing. Your calculations are correct}}$.

We should have;
$$\underbrace{secret}_{14-byte}\mathbin\|\underbrace{message}_{55-byte}\mathbin\|\underbrace{100\ldots00}_{51-byte \text{ padding 10 part}}\mathbin\|\underbrace{000\ldots 0228}_{8-byte \text{ length part}}$$

SHA-1 padding rule

append the bit $\texttt{1}$ to the message e.g. by adding byte $\texttt{0x80}$ if message length is a multiple of 8 bits i.e. byte-oriented.


append $0 \leq k &lt; 512$ bits of zeroes, such that the resulting message length in bits is congruent to $$−64 \equiv 448 \pmod{512}$$


append the original message length, as a 64-bit big-endian integer.
Thus, the total length is a multiple of 512 bits.


The real extension;
Wikipedia doesn't show the required padding after &amp;waffle=liege is appended. This is 13 bytes text and the size of the message will be $1024+13*8= 1128$. Therefore we will need 3 blocks for the extension that makes total of 1536 bits. 64 bits of this will be the new size encoding and this will need 344 bits of $1000.000$
$$\underbrace{secret}_{14-byte}\mathbin\|\underbrace{message}_{55-byte}\mathbin\|\underbrace{100\ldots00}_{51-byte \text{ padding 10 part}}\mathbin\|\underbrace{000\ldots 0228}_{8-byte \text{ length part}} \mathbin\|\underbrace{\text{&amp;waffle=liege}}_{13-byte \text{ extended message}} \mathbin\|\underbrace{100\ldots00}_{43-byte \text{ padding 10 part}} \mathbin\|\underbrace{000\ldots 0468}_{8-byte \text{ extended length part}} $$
In other view;
$$\text{SHA-1}(\text{secret_key}\mathbin\|\text{message}\mathbin\| \text{pad1}\mathbin\| 
 \text{appended_data} \mathbin\| \text{pad2})$$