I don’t know if you’ve noticed, but there is a trend these days for people to say ‘I was sat’ instead of ‘I was sitting’ and ‘I was stood’ instead of ‘I was standing’.

What they are doing is using the past participle when the present participle is needed. 

Some would say that this is a ‘non-standard grammatical form’, possibly originating in the north of England, while others would say it is wrong.

Examples:

×I was sat there all on my own until the others came.

×I was stood at the bus stop when a van came by and splashed me.

There is often an implied element of complaint, or sometimes of anecdote, when the above construction is used, but whatever the situation, the sentences should read:

I was sitting there all on my own until the others came.

I was standing at the bus stop when a van came by and splashed me.

English verbs have two participles as mentioned above:

1.       The present participle is used to form continuous (sometimes called progressive) tenses such as:

• present continuous:                                       I am sitting
• past continuous:                                              I was sitting
• present perfect continuous:                        I have been sitting
• conditional continuous:                                I would be sitting

2.       The past participle is used to form simple perfect tenses such as:

• present perfect:                                               I have sat
• past perfect:                                                      I had sat
• future perfect:                                                  I will have sat
• conditional perfect:                                       I would have sat

Examples of participle formation include:

verb             past simple          past participle             present participle

to say                    said                                       said                                        saying

to have                 had                                        had                                        having

to sit                      sat                                         sat                                          sitting

to stand               stood                                    stood                                    standing

to be                     was                                         been                                     being

to sing                  sang                                       sung                                      singing

to write                wrote                                    written                                 writing

to eat                    ate                                          eaten                                    eating

to see                   saw                                          seen                                      seeing

to finish               finished                                 finished                               finishing

to kiss                  kissed                                      kissed                                  kissing

Footnote:

I think some comedians, and maybe I’m thinking of Victoria Wood or Les Dawson used to start some of their jokes by saying, ‘I was sat sitting…’ or ‘I was stood standing…’, thus confusing the issue further by using both past and present participles together.